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

Eduqas GCSE Biology — 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 apply fertilisers to their fields to increase crop yields. Heavy rainfall can wash these fertilisers into nearby rivers and lakes.

Model answer (4 marks)

Fertiliser runoff from fields carries excess nitrogen and phosphorus into rivers, causing eutrophication.
1. The nutrients stimulate rapid growth of algae and aquatic plants (algal bloom).
2. The dense algal layer blocks sunlight from reaching submerged plants, so they die.
3. Bacteria decompose the dead algae and plants, consuming dissolved oxygen.
4. The oxygen level falls, fish suffocate and die from hypoxia.

Examiner tips

  • Use the word "runoff" or "leaching" to show how fertilisers reach water. Show the sequence: fertiliser → algal bloom → loss of light → plant death → bacterial decomposition → oxygen depletion. Mention fish suffocation as the final effect.
  • common_mistakes
  • :
  • Failing to link fertiliser runoff to eutrophication. Not explaining the role of bacteria in consuming oxygen. Using vague terms like "pollution" instead of specific processes.

Mark scheme (4 marks)

  1. Fertilisers are washed into rivers by rain (leaching/run-off)
  2. Algae/water plants grow rapidly (algal bloom)
  3. Algae block sunlight so aquatic plants below cannot photosynthesise and die
  4. Bacteria decompose the dead algae/plants, using up oxygen, so fish suffocate/die from lack of oxygen

Key terms in this question

fertilisers

Related

More Human impact on the environment questions

▶ Try answering this question with AI marking (free) →