Explain how the use of fertilisers on farmland can lead to the death of fish in nearby rivers.

WJEC GCSE Biology (Wales) — 1.6.3 Human impact on the environment · Explain · 4 marks · View as Markdown

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

Farmers often apply fertilisers to their fields to increase crop yields. Heavy rainfall can wash these fertilisers into nearby rivers and lakes.

Model answer (4 marks)

1. Rainfall washes excess fertilisers from fields into rivers (leaching/run‑off).
2. The added nitrogen and phosphorous stimulate rapid growth of algae and other aquatic plants (algal bloom).
3. Dense algal growth blocks sunlight, preventing photosynthesis in submerged plants, which then die.
4. Bacteria decompose the dead plants and algae, consuming dissolved oxygen and creating hypoxic conditions that suffocate fish.

Examiner tips

  • Use the exact sequence: leaching → algal bloom → light deprivation → oxygen depletion. Mention ‘nitrogen and phosphorous’ as key nutrients. Show cause–effect links clearly.
  • common_mistakes
  • :
  • Confusing fertiliser runoff with direct fish poisoning. Failing to mention oxygen depletion as the final cause of death. Using vague terms like ‘pollution’ instead of specific processes.

Mark scheme (4 marks)

  1. Fertilisers are washed into the river by rain (leaching/run-off)
  2. Fertilisers cause algae/water plants to grow rapidly (algal bloom)
  3. Algae block light, causing underwater plants to die
  4. Bacteria decompose the dead plants/algae, using up oxygen, so fish suffocate/die

Key terms in this question

fertilisers

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