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Eutrophication

Ecosystems and material cycles · Human impacts, biodiversity and food security · note 3 of 5

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EutrophicationSpec 9.9

Eutrophication happens when too many nutrients, such as nitrates from fertilisers or sewage, run off into lakes and rivers.

  1. Nitrates from fertilisers or sewage are washed into the water.
  2. Algae in the water grow very quickly because of the extra nutrients, forming a layer on the surface (an algal bloom).
  3. The layer blocks light, so plants and algae lower down cannot photosynthesise and die.
  4. Bacteria (decomposers) feed on the dead material and their numbers increase.
  5. The bacteria use up oxygen in the water for aerobic respiration.
  6. Fish and other aquatic animals die from lack of oxygen, so biodiversity falls.
Lake cross-section showing fertiliser run-off, an algal bloom blocking light, dead plants on the lake bed, bacteria using up oxygen and a dead fish, numbered to match a six-step flow chart of eutrophication.Tap to enlarge
Eutrophication: extra nitrates cause an algal bloom, plants die, decomposers use up the oxygen and fish die.