What causes eutrophication?

Eutrophication is a catchword that often comes up in the context of lakes and coastal waters. Why does it happen, and what does a lake affected by eutrophication look like?

Eutrophication is caused by nitrogen and phosphorus emissions in water, and high availability of these nutrients makes vegetation thrive on land and in water alike. Nutrients are carried into lakes both with wastewaters and runoff from fields and forests as well as transported by rivers from further upstream.

Eutrophication typically affects water areas surrounded by fields or located near urban centres. Small shallow lakes with a long water retention time are particularly prone to eutrophication.

The signs of eutrophication are many: algae tend to proliferate, and in mid-summer you may see blue-green algae blooms on the surface. The water becomes opaque and its transparency is reduced, and shoreline rocks and fishing nets get slimy.

Plant and animal communities change and become less diverse, and reed beds spread and may take over the whole shore. Cyprinids proliferate while more valuable fish are reduced in numbers. At worst, the lake may be hit by oxygen depletion and fish kills.