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

Pearson Edexcel International GCSE Biology (4BI1) — 4.4 Human influences 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 nitrate fertilisers to their fields to increase crop yields. Rain can wash these fertilisers from the soil into nearby bodies of water.

Model answer (4 marks)

Fertiliser runoff from farmland carries excess nitrate into the lake.

The nitrate stimulates rapid growth of surface algae (algal bloom).

The dense algae layer blocks sunlight, preventing photosynthesis in submerged plants.

When the algae and plants die, decomposers consume the organic matter, depleting dissolved oxygen and causing fish to suffocate.

Examiner tips

  • Use the exact sequence: runoff → algal bloom → light limitation → plant death → oxygen depletion → fish death.
  • Show cause–effect links and use terms like ‘run‑off’, ‘algal bloom’, ‘photosynthesis’, ‘decomposers’ and ‘dissolved oxygen’.
  • Keep each point short and to the point, matching the four marks.

Common mistakes

  • Mixing up nitrate with phosphate or using ‘fertiliser’ instead of ‘nitrate’
  • Failing to mention light blockage or oxygen depletion
  • Writing a long paragraph instead of separate points

Mark scheme (4 marks)

  1. Fertilisers are washed from the land into the lake (leaching / run-off)
  2. Fertilisers cause excessive growth of algae at the water surface
  3. Algae block light from reaching aquatic plants, so aquatic plants die / cannot photosynthesise
  4. Decomposers break down the dead plant / algae material, using up oxygen, so fish die from lack of oxygen

Related

More Human influences on the environment questions

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