Regime Shifts DataBase

Large persistent changes in ecosystem services

Eutrophic Baltic Sea: reinforcing feedbacks

• Hypoxia or anoxia (Local, well-established). Hypoxia and anoxia affect nutrient transformation processes (nitrification, denitrification) and the capacity of the sediments to bind phosphorus. In the absence of oxygen, decomposing sediments release significant quantities of phosphorus into the water. The ability of the ecosystem to lose nitrogen through denitrification is limited to regions with low oxygen concentrations. The increased phosphorus and nitrogen concentrations (see below: cyanobacteria) accelerate the rate of eutrophication. This feedback creates a persistent internal loading of phosphate even if external nutrient loads are reduced.

• Cyanobacteria (Local, well-established). Parts of the Baltic marine ecosystem are trapped in a feedback mechanism encouraging algal blooms although the inputs of nitrogen and phosphorus have been reduced. Anoxia facilitates the release of phosphorus from the sea floor sediments, fueling the growth and blooms of cyanobacteria. Some cyanobacteria species can fixate nitrogen gas. In addition, especially during their bloom in the late summer, cyanobacteria may release nitrogen compounds, which can partly be used by other organisms. 

• Anthropogenic loads of nitrogen and phosphorus (Regional, well-established). The increased nutrient load has been a major driving force behind the oxygen deficiency as it has increased algal production and hence sedimentation of organic matter to the deepwater. Nitrogen and phosphorus are nutrients, which critically determine the growth of primary production in the Baltic Sea. The main sources of anthropogenic inputs of nitrogen and phosphorus to the Baltic Sea are derived from agriculture, industry, municipal sewage and atmospheric depositions.

• Water inflow from the North Sea (Regional, well-established). Deep-water oxygen concentrations in the Baltic Sea are influenced by eutrophication and saltwater inflows from the North Sea. The limited water exchange with the North Sea and the long residence time of water are the main reasons for the sensitivity of the Baltic Sea to the eutrophication: nutrients discharged into the sea will remain in the basin for a long time. The inflowing deep, dense saline water replaces the old stagnant water in the deeps below the halocline. The displaced stagnant nutrient rich water is brought into the productive surface layer via upwelling (natural internal loading). Vertical stratification of water masses increases the vulnerability of the Baltic Sea to the eutrophication as it hinders or prevents ventilation and oxygenation of the bottom waters and sediments. It appears that there has been only a few MBIs since the mid-1970s, which is believed to have resulted in a long-term stagnation of the deepest bottom water. North Sea saltwater inflows are governed by large-scale and local meteorological variations and by sea level and salinity distributions.

• Decrease in wetlands (Regional, well-established). Extensive draining of wetlands and lakes has increased the proportion of agricultural nutrients that is transported to the Baltic Sea. Also drainage of cropland decreased water retention in the landscape, increased the agricultural productivity, but increasing the speed with which water (and nutrient) ran off the landscape into the Baltic. 

Shift from oligotrophic to eutrophic Baltic Sea

• Intense algal growth – the threshold of which decomposition of the abundant algal biomass leads to oxygen deficiency and hydrogen sulphide production.

• Anoxic deepwater conditions – the threshold of which the living conditions become intolerable for fish and benthic fauna.

• Nutrient load – the threshold of water nutrients at which algal blooms occur. 

• Nutrient inputs (Regional, well-established). Nitrogen and phosphorus are the nutrients, which critically determine the growth of primary production in the Baltic Sea. The input of phosphorus into the Baltic is approximately eight times greater and nitrogen input approximately four times greater today than in the beginning of the 20th century. To actively decrease nutrient inputs form agriculture, urban and rural wastewaters and aquaculture, reducing the use of artificial fertilizers, preventing diffuse nutrient losses from fields and animal husbandry, and minimizing nitrogen emissions to air from shipping, animal husbandry and land-based traffic.

• Hypoxia/anoxia (Local, speculative). Pilot studies aimed at artificially oxygenating deep-water basins to combat oxygen deficiency are carried out. Hypoxia and anoxia make deepwater habitats unsuitable for fish and benthic fauna. Anoxia has been pointed out as a reason for cod stock decline, since the early stage cod can't survive in low oxygen concentrations. 

It is difficult to define the impacts of the Baltic Sea eutrophication on ecosystems because of the large natural spatial and temporal variation in the Baltic Sea, but certain changes can be attributed to anthropogenic eutrophication: increase in primary production, increased occurrence of algal blooms, changes in the species composition of plants and animals and spreading of dead zones (anoxic areas).

Regulating services

Abundant phytoplankton species generate excess photosynthetic production, which the Baltic Sea ecosystem is unable to process. Algal mats sink down to the seafloor and decompose, depleting the oxygen in the near bottom water layer. In the last two decades, up to a third of the deep seafloor has been anoxic at the height of the stagnation. In the mid-1990s the area was reduced due to a saline water inflow, but the anoxia returned with time. In the 2000s, the largest area of seafloor ever recorded was anoxic and hydrogen sulphide concentrations were high. Large areas of the Baltic seafloor have become devoid of life due to the mortality of the benthic fauna caused by anoxia and hypoxia. Hypoxia eliminated bottom fauna over vast areas below the halocline already in 1958-59. Additionally, when the organic biomass decomposing conditions become anoxic, the decomposing bacteria acting on these conditions start to produce hydrogen sulphide, which is extremely toxic. Gradually the living conditions become intolerable for fish and the benthic fauna. Fish move away and benthic fauna dies in masses, resulting in large areas of dead zones in the sea floor. Also, the increase in phytoplankton leads to greater turbidity and thus decreased light penetration, which limits the habitat available for macrophytes, such as Fucus.

Provisioning services

Deepwater anoxia has affected the success of cod reproduction, which has in part caused the change in Baltic food web from a cod-dominated regime to sprat-dominated. When it comes to eutrophication induced changes in the Baltic species, it has been estimated that the phytoplankton biomass has increased by 30-70% since the beginning of the 20th century. Zooplankton increased 50% between 1951-1960. The abundance of shallow-water bottom-living macrofauna increased 3-5 fold during 1920-76 but the biomass of the benthic macrofauna in open sea decreased. Fish populations have become more abundant during the last century, but it cannot be entirely associated to eutrophication as there are other factors affecting the biomass of fish as well.

Increased sedimentation of organic matter has benefited some algal species while perennial species, for instance Bladder wrack (Fucus vesiculosus) have declined. Bladder wrack, an important nursery environment for fish and many invertebrates, has suffered from the lack of light. On the contrary, filamentous algae (e.g. Cladophora glomerata) has thrived on high nutrient conditions. The filamentous algae support large populations of juvenile invertebrates, which later on move down to feed on the bladder wrack causing damage by increased grazing pressure. Disappearance of the Bladder wrack causes the disappearance of the diverse faunal community, which it supported.

There is some evidence that Baltic populations are genetically differentiated to live in brackish water, which increases the importance of protecting the locally adapted stocks, since they may be impossible to replace if lost. One example of such species is eastern Baltic cod, which is important to the Baltic Sea both ecologically and socio-economically.

Together with overfishing, eutrophication has the largest negative impact on the environmental state of the Baltic Sea. Ecosystem services, habitats, food and tourism are all threatened by eutrophication.

Cultural services

Algal blooms have led to closed beaches and caused frequent public concerns. The growth of algae in the form of slime, mats and blooms near shores and shallow areas and unpleasant tastes and odors result in harmful effects on water use, e.g., recreation activities, aesthetic quality and commercial and sport fishing. A low water quality also lowers the value of coastal properties.

Algal blooms clog municipal and industrial water supply filters resulting in large cleanup costs. They have also been associated with seafood contamination by neurotoxins leading to human health problems. For example, mussel farms in Sweden affected by toxic algae has occasionally been forbidden to sell their harvest.

Large economic costs also result from the loss of revenue from tourism and recreation. Although all tourism is not based on marine environment, the Baltic Sea environment and its cultural fishing surroundings are important as tourist attractions. 

In the 2000s, large parts of the Baltic Sea experienced the worst ever hypoxia. In recent years the occurrence of moderate hypoxia has further increased, which has been suggested to be likely caused by eutrophication.

Finding solutions to reduce eutrophication is probably the most difficult challenge for the Baltic Sea future. The success of the HELCOM Baltic Sea Action Plan depends on the co-operation of all the coastal countries. All major stakeholders (governments, NGOs, industry, citizens etc.) are included. The targets for the good ecological status of the Baltic Sea are clearly defined and based on the best available scientific knowledge and ecosystem approach. The management goal is a healthy, biologically well-balanced marine environment, which can support several sustainable human activities. The nutrient and oxygen levels should become natural and water clear, algal blooms occur on natural scale only, and the natural distribution of plants and animals should be unaffected.

There is a debate going on about whether the Baltic Sea eutrophication regime shift is irreversible or not. HELCOM continues to observe the Baltic Sea eutrophication status, which provides a good basis for evaluating the effectiveness of the management efforts. According to the HELCOM 2009 report, the measures taken for decreasing the phosphorus and nitrogen inputs are effective but there will be a time lag until the results can be seen. The effect of phosphorus reduction has been noticeable only in the Bothnian Bay. Nutrient inputs need to be further reduced and many problematic nutrient sources still remain, e.g. the wastewater treatment facilities of St.Petersburg and agriculture and industries of countries undergoing economic transition.