Regime Shifts DataBase

Large persistent changes in ecosystem services

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr^-1 under shelf interiors, and from 5 to 10 m yr^-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO₂ uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr^-1 under shelf interiors, and from 5 to 10 m yr^-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO₂ uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr^-1 under shelf interiors, and from 5 to 10 m yr^-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO₂ uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr^-1 under shelf interiors, and from 5 to 10 m yr^-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO₂ uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr^-1 under shelf interiors, and from 5 to 10 m yr^-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO₂ uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr^-1 under shelf interiors, and from 5 to 10 m yr^-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO₂ uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr^-1 under shelf interiors, and from 5 to 10 m yr^-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO₂ uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr^-1 under shelf interiors, and from 5 to 10 m yr^-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO₂ uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr^-1 under shelf interiors, and from 5 to 10 m yr^-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO₂ uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits.