Download4 Eutrophication

What are the problems?

Eutrophication affects marine ecosystems in many ways

Eutrophication is mainly a problem in coastal areas and in areas with restricted water exchange, such as enclosed estuaries and embayments. Eutrophication causes changes in the composition of plant and animal communities and generally favours growth of rapidly reproducing opportunistic algal and animal species Figure 4.1. Opportunistic algal species do not always pose a threat but some species can adversely affect ecosystems. Mass occurrence of phytoplankton also reduces the depth at which light is available for long-lived seagrass species. Once the nutrients have been depleted, the algal blooms associated with nutrient enrichment decay, leading to oxygen deficiency and possibly kills of fish and benthic invertebrates, and to the formation of toxic hydrogen sulphide (H2S).

As well as causing impacts on the ecosystem, eutrophication can affect human activities. For example, algal blooms can clog fishing nets. Decaying blooms of some algae can create unsightly foam masses on beaches and unpleasant smells that interfere with tourism and recreation. Although some algae produce toxins that can harm humans through consumption of contaminated shellfish, the link to nutrient enrichment is uncertain.

Excess nutrients result from sources on land and at sea

Rivers are the main pathway for excess nutrients to Regions II, III and IV, collecting direct discharges from point sources, such as sewage treatment plants and industry, and inputs from land run-off and leaching, mainly as a result of agriculture. The amounts of nutrients released from land-based sources vary according to land use and population density. Point sources generally dominate in urban areas, while diffuse sources dominate in farming areas. Farmland covers about half the total land area in many OSPAR countries, reaching 60% to 70% for some countries bordering Regions II and III. The rate of turnover of nutrients in soils and sediments means they can be released to the marine environment for decades after releases from the original sources have been reduced. A further potential confounding factor is hazardous substances, some of which (e.g. certain anti-fouling agents) have the potential to influence algal growth and in turn eutrophication effects. Transboundary transport of nutrients by ocean currents is particularly important in Region II.

Atmospheric deposition is an important pathway for nitrogen to the sea and is usually greatest close to the source. Nitrogen is emitted to the atmosphere from agriculture and from combustion processes associated with industry and transport, including maritime shipping, and can be carried by winds to places far from the emission sources, where it is deposited. In Regions I and V, far from most point sources of pollution, atmospheric deposition is the main pathway for anthropogenic inputs of nitrogen.

Climate change may alter impacts

More rain and increased flooding as a result of climate change are expected to enhance nutrient enrichment through increased freshwater input and run-off from land Chapter 3. Rising sea temperature and prolonged stratification are likely to lead to increased incidence of harmful algal blooms and changing phytoplankton composition. Ocean acidification may also promote changes in the plankton. Recent observations of the decline in sugar kelp along the southern coast of Norway indicate possible interactions between climate change and eutrophication. Improved understanding of this interaction will be important in OSPAR’s future work on eutrophication.

Figure 4.1 Sources of nutrient input to the marine environment...

Box 4.1 Declining sugar kelp forests on the Norwegian coast

Surveys of sugar kelp forests between 1996 and 2006 by the Norwegian Institute for Water Research uncovered a dramatic decline in their abundance along the Norwegian coasts. Abundance of sugar kelp reduced by 40% on the western coast and 80% on the southern (Skagerrak) coast. The decline was most pronounced in sheltered waters, where the sugar kelp forests have been replaced over large areas by a silty turf community dominated by filamentous algae.

The shift in vegetation probably reflects a combination of long-term eutrophication (transboundary inputs as well as local nutrient inputs) and recent climatic events resulting in higher sea temperatures. The decline in sugar kelp followed exceptionally warm summers. Eutrophication may also have negatively affected recruitment of new plants on the Skagerrak coast.

The kelp forests are highly productive and diverse communities, providing habitat for many organisms, and are important feeding and nursery areas for many species of fish. In contrast, the turf communities provide much less food and shelter. The ecological and economic consequences of these changes within the coastal zone are uncertain. Estimates suggest that the lost sugar kelp forests mean a shortfall of 50 000 tonnes of fish biomass and CO2 capture capacity worth 11 million Euro (based on 18 Euro per tonne CO2).

The impact of climate change on eutrophication status