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Descriptor 5: Eutrophication

“Human-induced eutrophication is minimised, especially adverse effects thereof, such as losses in biodiversity, ecosystem degradation, harmful algae blooms and oxygen deficiency in bottom waters”

What is eutrophication?

Eutrophication is a process driven by the enrichment of water by nutrients, especially compounds of nitrogen and/or phosphorus, leading to: increased growth, primary production and biomass of algae; changes in the balance of organisms; and water quality degradation. The consequences of eutrophication are undesirable if they appreciably degrade ecosystem health and biodiversity and/or the sustainable provision of goods and services.  

Nitrogen and phosphorous are the primary inorganic nutrients responsible for the eutrophication of marine waters. Nitrogen and phosphorous occur naturally in marine waters, transferred from land via streams, rivers and runoff of rainwater and also from degradation of organic material within the water. However, human inputs of nutrients to the environment has increased the load of nitrogen and phosphorous to the oceans.

logo marine directiveThe Marine Directive requires the Members States to reduce the nitrogen and phosphorous loads to the marine environment and thereby to reduce eutrophication. Descriptor 5 states that the human-induced eutrophication should be minimized, and in particular the adverse effects it has on the marine environment, such as losses in biodiversity, ecosystem degradation, harmful algae blooms and oxygen deficiency in bottom waters.

What are the main sources of eutrophication?

intensive culturesThe predominant nitrogen load comes from diffuse sources on land, especially agricultural areas. Other sources of nitrogen include nitrogen gases (e.g. ammonium from manuring, nitrogen oxides from ships, which are transferred via the atmosphere to oceans through precipitation), aquaculture, waste water treatment plants, industrial water and adjacent oceans.

The main contribution of human-introduced phosphate comes from domestic and industrial sewage and waste water. Run-off from land is also an important source of phosphate as it is a component of fertilizers.

Why should we pay attention to eutrophication?

Primary Impacts

Algal growth in marine waters is regulated by the level of nitrogen and phosphorous and to a lesser extent other inorganic compounds. Eutrophication leads to an increased algal growth (because the level of nutrients increases). It can lead to a shift in species composition to fast growing algae species (including toxic species) and a shift from long lived macroalgae to more nuisance species.

Secondary Impacts


Eutrophication leads Marée Verte - IFREMER

Secondary impacts from large algal blooms can lead to various effects throughout the ecosystem.

  • The reduced amount of sunlight received by bottom waters can lead to the reduction in the depth distribution of macroalgae and sea grasses.
  • Increased decomposition of organic matter (dead algae) can lead to oxygen deficiency in bottom waters. Lowered oxygen concentrations can then impact the fish and benthic fauna (animals living on the bottom of the sea or a lake), which either flee or die from the area.
  • In the end, eutrophication can cause a shift in the biodiversity and ecosystem balance.

It can also have socio-economic impacts:

  • Oxygen depletion can reduce fish and shellfish stocks and thereby have an economic impact on the fishery industry.
  • Algal toxins from harmful algal blooms can cause shellfish poisoning in humans and be of danger to live stocks in coastal water.

The water quality can be reduced due to decaying algae with foul odours and foam on beaches, or toxins from blooms, impacting the tourism industry.

What can be done?

fqsd In order to reduce human induced eutrophication, it is necessary to reduce the nitrogen and phosphorous load to the oceans which can be done through changes in the agricultural practices, for example by restrictions in the use of fertilizers, optimizing nutrient use to crop requirements, planning of the use of fertilizers, so washout from land is minimized, establishment of more sustainable agriculture farms.

Furthermore reductions in atmospheric sources of nitrogen, better cleaning of sewage and waste water, better control of diffuse urban nutrient sources such as runoff from streets and storm sewers and introduction of wetlands as nutrient sinks can be mentioned as some of the solutions to the eutrophication issues.

For many years now, the EU has been taking steps towards the reduction of nitrogen and phosphorous loads in the environment, notably through the adoption of several crucial pieces of legislation:

  • The Nitrates Directive (1991) aims to protect water quality across Europe by preventing nitrates from agricultural sources polluting ground and surface waters and by promoting the use of good farming practices.
  • The Urban Waste-Water Treatment Directive (1991) aims to protect the environment from the adverse effects of urban waste water discharges and discharges from certain industrial sectors.
  • The Water Framework Directive (2000) lays down a strategy to fight against the pollution of water, including adopting specific measures against pollution by individual pollutants or groups of pollutants presenting a significant risk to or via the aquatic environment.



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