Describe the process of eutrophication, including how it leads to a reduction in the biodiversity of a river.

Cambridge International IGCSE Biology (0610) — 20.3 Pollution · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Fertilisers washed from fields enter the river, giving an excess of nutrients. This causes algae and aquatic plants to grow rapidly, forming an algal bloom that covers the water surface and blocks sunlight. Aquatic plants below the surface cannot photosynthesise and die; decomposers break down the dead plants and consume oxygen. The resulting decrease in dissolved oxygen causes organisms such as fish to die, reducing biodiversity.

Examiner tips

Use the exact sequence: fertilisers → excess nutrients → algal bloom → light block → plant death → decomposition → oxygen depletion → fish death.
Mention the link between oxygen loss and biodiversity loss.
Keep the answer concise and use the terminology from the mark scheme.

Common mistakes

Confusing eutrophication with general pollution; not mentioning the nutrient source.
Failing to explain the oxygen depletion step or its effect on fish.
Writing too much detail about algae types instead of the key process steps.

Mark scheme (4 marks)

Fertilisers are washed from fields into the river, providing an excess of nutrients
Algae / aquatic plants grow rapidly, forming an algal bloom that covers the water surface and blocks sunlight
Aquatic plants below the surface cannot photosynthesise and die; decomposers break down the dead plants and use up oxygen
The decrease in dissolved oxygen causes organisms such as fish to die, reducing biodiversity

Key terms in this question

eutrophication · biodiversity