Supplement 3.3: Eutrophication

What is Eutrophication?

Nutrients such as nitrogen and phosphorus are essential for maintaining primary production and thus the healthy structure and functioning of aquatic ecosystems.

The process where bodies of water, such as lakes, estuaries, or slow-moving streams, receive excess nutrients that stimulate excessive plant growth is called eutrophication.

This enhanced plant growth, often called an algal bloom, reduces dissolved oxygen in the water (anoxia). This can kill other marine life which also depend on disolved oxygen in the water.

Phytoplankton bloom in the Baltic Sea
A colourful summer marine phytoplankton bloom fills much of the Baltic Sea in this image captured by Envisat's MERIS on 13 July 2005.
Source: ESA

What causes eutrophication? - Sources of nutrients

In north-western Europe and the Danube river catchment, 50-80% of the total load of nitrogen originates from diffuse pollution from agricultural run-off (EEA 2005).

True colour image of the Mississippi delta
This true colour satellite image displays the alluvial fan of the Mississippi delta where the river flows into the Gulf of Mexico. The effluents deposited at the delta can be clearly seen. Those deposits include topsoil as well as nitrogen-based farm fertilizer runoff, which depletes the surrounding water of oxygen.
Source: NASA

The main sources of phosphorous used to be industry and household wastewater. However, due to legally enforced reductions in point-source discharges since the 1980s, in some countries agriculture has also become the main source of phosphorous contribution to the Eropean seas (EEA 2007).

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