Primary Production in the Arctic Ocean
A shift from polar to temperate primary production (PP) patterns has been detected in the Arctic Ocean. Following a regime shift in the North Atlantic in 1995, similar structural changes are now occurring in Arctic waters. Rapid warming of atmospheric and oceanic temperatures has caused a near year-on-year decline in the extent and thickness of summer sea ice since 1979 (NSIDC 2014). Anthropogenic climatic change has extended the growing season and delayed August freeze-up through a decline in albedo reflectivity and enhanced wind-driven vertical mixing. Natural modes of variability at the lower latitudes has also led to poleward shifts of temperate marine species and caused pronounced phenological changes to primary producers. The difference in the temporal scale of these forcing mechanisms makes it hard to predict which event is causing changes to PP. It is uncertain what impact this change will have on the food web of this ecosystem.
Key direct drivers
- External inputs (eg fertilizers)
- Environmental shocks (eg floods)
- Global climate change
Key Ecosystem Processes
- Primary production
- Nutrient cycling
- Wild animal and plant products
- Climate regulation
- Natural hazard regulation
- Aesthetic values
- Knowledge and educational values
- Food and nutrition
- Health (eg toxins, disease)
- Livelihoods and economic activity
- Cultural, aesthetic and recreational values
Typical spatial scale
Typical time scale
- Contemporary observations
Confidence: Existence of RS
- Contested – Reasonable evidence both for and against the existence of RS
Confidence: Mechanism underlying RS
- Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms
Links to other regime shifts
Anthropogenic climate change is largely responsible for the observed changes in the Arctic Ocean over the last few decades (Dicks et al. 2011). The Arctic thermal, hydrological, climatological and biological changes are highly interconnected and the system dynamics are poorly understood insulated from each other (Hinzman et al. 2005). Precisely how the changes in the biological conditions will affect the food web system in the Arctic Ocean is unknown; however, it is evident that people depending on Arctic fish for their livelihoods will be effected in the short term (ibid), while in the longer term, changes might profoundly affect commercial fish stocks in the lower latitudes (Hassan et al. 2005).
The polar regime is characterized by long winters of 7-10 months (Hinzman et al. 2005). The large extent of year-round sea ice reinforce the albedo effect exhibited at these latitudes, keeping the system in a state of low primary productivity. The ice melt in summer allows phytoplankton to reproduce and provide energy for the dominant cold-loving zooplankton species Calanus hyperboreus and C. glacialis (Ardyna et al. 2014).
Summer air temperatures over the Arctic have continued to increase since 1800 (Dicks et al. 2011). The associated decline in sea ice extent and thickness has allowed light to penetrate the upper water column, while the exposure of year-round sea water has increased storminess and the upwelling of nutrients. In some cases, this has led to two annual bloom events and an extent of single annual blooms in areas previously characterized by flat patterns at the sea-ice boundary (Ardyna et al. 2014).
Drivers and causes of the regime shift
Global warming is well documented as the main driver of warmer air temperatures over the Arctic (Dicks et al. 2011). Warming is causing sea ice to melt and is extending the amount of year-round and seasonal open water (Ardyna et al. 2014). This darker surface is reducing albedo reflexivity and increasing sea surface temperatures (SST). The IPCC AR5 states that warming must be kept below 1°C to prevent irreversible changes of the Arctic system.
Natural variations in Arctic SST are observed through Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) internal forcing mechanisms, both modes of climate variability that originate in the lower latitudes. The latter enhances westerly winds, while the combination of both the NAO and a positive AMO enhance ocean advection and storm events (Schlesinger & Ramankutty 1994). This phase of activity has been observed since the mid-1990s, enhancing the poleward movement of warm, saline waters to the eastern Arctic (Chylek et al. 2009).
The amount of water masses entering into the Arctic is highly influenced by the strength of Subpolar gyre (Hátún et al. 2009), which itself is influenced by the AMO mode. Since 1995, the North Atlantic Subpolar gyre has been weakening (Häkkinen & Rhines 2004). Together, the AMO and Subpolar gyre have enhanced the observed melting of sea ice in the Arctic and the calving events of the Greenland Ice Sheet (GIS) (Straneo & Heinback 2009). However, freshwater inflow from Greenland and Siberian runoff reduces the density of the water column and enhances stratification with the denser saline layer below. It is uncertain how this inflow might counter the observed increase in bloom events, but a change in Siberian forest cover could enhance nutrient inputs and thus promote further PP, where rates of PP have already increased by 135% from 1998 to 2009 (Frey et al. 2007).
How the regime shift works
The polar region is characterized by relatively low rates of precipitation and two distinct seasons (Hinzman et al. 2005). The winter season, lasting for 7-10 months, displays a large sea-ice extent, maintained through an ice-albedo feedback (Curry et al. 1995). During the summer, the sea ice retreats and allows for PP to occur in areas where light can penetrate the water column (Ardyna et al. 2014). These conditions are especially beneficial for the cold-loving zooplankton species Calanus hyperboreus and C. glacialis. They successfully convert phytoplankton into energy-dense marine lipid, and thus play a vital role in sustaining the higher trophic levels of the Arctic ecosystem (Falk-Petersen et al. 2007). In September the annual freeze-up begins.
The ocean and atmosphere work together to drive currents around the planet. At the interface, this heat flux causes the development of trade winds and storm events. This interaction is amplified as the sea ice melts and - coupled with warmer air temperatures - it increases storminess and vertical mixing in the upper layers of the water column. Such turbulence brings nutrients and heavier phytoplankton to the surface, promoting suitable conditions for PP. This coupling has intensified with more ice-free waters in the summer months and led to an upward trend in the length of the growing season (Frey et al. 2007). Since 1995, the positive modes of variation along with a weak Subpolar gyre and strong density-driven Thermohaline conveyor (THC), have favoured the poleward movement of warmer waters (Hátún et al. 2009), bringing warm-water-loving species (eg. herring) north and contracting the range of cold-water-loving species, such as the zooplankton C. finimarchicus, northwards. These mechanisms are amplifying the effect of anthropogenic forcing in the Arctic Ocean but it is unsure how they, in turn, will be affected by climate change in the future (Hátún et al. 2005).
The temperate regime is characterised by two annual phytoplankton bloom events. The Arctic Ocean is the least productive of all the oceans, but as permanent ice cover and stratification has declined, primary productivity has increased (Ardyna et al. 2014). A delay in the annual freeze-up event due to warmer SST has lengthened the growing season. Phenological changes in the timing of blooms, advancing by as much as 50 days between 1997 to 2009, has allowed for double bloom events characteristic of more temperate climates and a compositional shift towards the dominance of smaller phytoplankton species (Li et al. 2009). Earlier melting is increasing nutrient-rich freshwater inputs from coastal shelves, such as Siberia and Greenland, while oceanic systems are driving nutrient-rich waters from the lower latitudes (Ardyna et al. 2014). Together, these processes act to maintain the Arctic in a temperate state.
Impacts on ecosystem services and human well-being
Shift from polar regime to temperate regime
Ice-dependent species with limited distribution, specialised feeding habits and predator avoidance (Post et al. 2009), such as cod, shrimp, marine mammals and seabirds, have seen changes in abundance due to warmer temperatures and changes in their ecosystems (Wasmann 2011). In contrast, warm-loving species are thriving due to rising SST and food abundance (Hátún et al. 2009). Nevertheless, in the long term a shift in food-web composition and diversity could diminish the resilience of the Arctic system by the invasion of non-native species, eutrophication or the spread of disease thus decreasing the recreational and aesthetic value of these ecosystems (Niiranen et al. 2014).
Due to their highly adapted way of life, the loss of provisioning ES could indirectly impact indigenous wellbeing and livelihoods (Arctic Council 2013). Contrarily, this shift to a temperate regime could bring shorter-term gains to tourism (eg. whale watching) and commercial fishing, helped by easier navigation in ice-free waters, although the benefit will be felt by mainly non-Arctic residents (Hassan et al. 2005). Human health is at risk due to the consumption of high level fish and mammal from the Arctic by the bioaccumulation of pollutants (Butler and Oluoch-Kosura 2006). Changes in the Arctic will also have spillover effects in Atlantic waters, reducing PP and food provisioning (Arctic Council 2013).
From the point of view of PP, it is unclear how the regime shift would affect the ecosystem and the services people derive from it (Moore & Huntington 2008; Greene et al. 2008; Li et al. 2011). However, the regime shift is connected to other changes with possibly catastrophic implications, such as reduced Albedo negatively influencing the Arctic Ocean’s temperature regulating ability, or the loss of habitat for Arctic-specific species. Considering these implications of a regime shift, from a global perspective polar conditions are clearly preferable.
It is essential to cut global emissions of greenhouse gases (GHG) in order to prevent further sea ice loss and the kicking-in of feedback mechanisms that risk putting the Arctic in a permanent temperate state. Equally important is to prevent deforestation, as this reduces the biosphere’s ability to sequester atmospheric carbon. Especially important, in the polar region, ceased deforestation could reduce primary production by decreasing the discharge of nutrients into the Arctic Ocean. For carrying this out, international agreements on emissions and land use are necessary. However, it might not be enough to prevent a regime shift considering observed changes and the time lag of global climate processes. Management options for reversing the regime shift are essentially the same as for preventing it, but a reversal is likely to take a long time, as several feedback mechanisms will drive the system towards temperate conditions.
Although highly criticised, geoengineering has been proposed as a way to speed up the reversal of the regime shift (Heckendorn et al. 2009). A scheme often put forward is to reflect some solar radiation back into space by introducing aerosols into the stratosphere. Though economically viable, this is highly controversial due to its ethically challenging nature, the many possible ecological side effects and the impossibility to reverse action once implemented.
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