Eutrophication

Unique identifier / Notation

EN0024

Synonyms

None identified.

Eutrophication is the overabundance of nutrients in a body of water that results in harmful algal blooms, fish kills, and in some cases ecosystem collapse. It is a process driven by enrichment of water by nutrients, particularly compounds of nitrogen and/or phosphorus, leading to increased growth, primary production and biomass of algae; changes in the balance of nutrients causing changes to the balance of organisms; and water quality degradation (NOAA, 2007; UNEP, 2015).

Primary reference(s)

NOAA, 2007. National Estuarine Eutrophication Assessment: Update. National Centers for Coastal
Ocean Science, National Oceanic and Atmospheric Administration (NOAA). Accessed 14 October 2020.

UNEP, 2015. Mediterranean Action Plan. Report of the Online Groups on Eutrophication, Contaminants and Marine Litter. United Nations Environment Programme (UNEP). Accessed 14 October 2020.

Additional scientific description

Eutrophication is the nutrient output (mainly nitrogen and phosphorus), such as from sewage outfalls and fertilised farmland, that accelerates the growth of algae and other vegetation in water. The degradation of organic material consumes oxygen resulting in oxygen deficiency and, in some cases, fish death. Eutrophication translates the quantity of substances emitted into a common measure expressed as the oxygen required for the degradation of dead biomass (FAO, 2017).

Eutrophication resulting from excess inputs of nutrients from both agriculture and sewage causes algal blooms. Those can generate toxins that can make fish and other seafood unfit for human consumption. Algal blooms can also lead to anoxic areas (i.e., dead zones) and hypoxic zones. Such zones have serious consequences from environmental, economic and social perspectives (United Nations, 2017).

Where there are narrow continental shelves, some wind conditions can bring nutrient-rich, oxygen-poor water up into coastal waters, and produce hypoxic (low-oxygen) or even anoxic (no-oxygen) conditions and eutrophication can develop. Changes in ocean circulation appear to be enhancing those effects. Examples of this can be found on the western coasts of the American continent immediately north and south of the equator, the western coast of sub-Saharan Africa and the western coast of the Indian subcontinent (United Nations, 2017).

Marine biota are subject to many different pressures from hazardous substances, including the impact of such substances on reproductive success. Dead zones and low-oxygen zones resulting from eutrophication and climate change can lead to systematic changes in the species structure at established fishing grounds. Either can reduce the extent to which fish and other species used as seafood will continue to reproduce at their historical rates. When those effects are combined with those of excessive fishing on specific stocks, there are risks that the traditional levels of food provision from the sea will not be maintained (United Nations, 2017).

Metrics and numeric limits

It is unclear if there is a standard global metric for measuring the effects of eutrophication.

Key relevant UN convention / multilateral treaty

Not found.

Examples of drivers, outcomes and risk management

Excessive nutrient levels (eutrophication) lead to anoxic and hypoxic zones in the aquatic environment. The hypoxic and anoxic zones created by eutrophication drive fish away and kill the benthic wildlife. Where those zones are seasonal, any recovery is usually at lower trophic levels, and the ecosystems are therefore degraded. This seriously affects the maritime economy, both for fishermen and, where tourism depends on the attractiveness of the ecosystem (for example, around coral reefs), for the tourist industry too. Social consequences are then easy to see, both through the economic effects on the fishing and tourism industries and in depriving the local human populations of food.