Describe the effects of eutrophication on the organisms living in a lake.

Eduqas A-Level Biology — 3.6 Human impact on the environment · Describe · 5 marks · View as Markdown

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

Farmers sometimes apply fertilisers to their fields. Rain can wash nitrates and phosphates from the soil into nearby lakes, causing eutrophication.

Model answer (5 marks)

Rapid growth of algae (algal bloom) occurs.
Algae block light from reaching submerged plants.
Submerged plants die because they cannot photosynthesise.
Decomposing bacteria break down the dead organisms and consume oxygen.
Fish and other aquatic animals die due to lack of oxygen.

Examiner tips

Use the exact terms: algal bloom, submerged plants, decomposing bacteria, lack of oxygen.
Show the causal chain: nutrients → algae → light block → plant death → bacterial decomposition → oxygen depletion → fish death.
Keep each point brief and separate with line breaks or numbering.

Common mistakes

Mixing up the order of events; students often list fish death before plant death.
Using vague words like ‘pollution’ instead of ‘nitrates/phosphates’ or ‘oxygen depletion’.

Mark scheme (5 marks)

Nitrates/phosphates cause rapid growth of algae (algal bloom)
Algae block light from reaching plants below the water surface
Submerged plants die because they cannot photosynthesise
Decomposing bacteria break down the dead organisms and use up oxygen
Fish and other aquatic animals die due to lack of oxygen

Key terms in this question

eutrophication