Explain how the use of fertilisers on farmland can lead to the death of fish in nearby rivers and lakes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Farmers apply fertilisers to crops to increase yields. When it rains, some of these fertilisers can be washed off the land into nearby bodies of water.
Model answer (5 marks)
Fertiliser runoff introduces excess nitrates and phosphates into rivers and lakes. The added nutrients stimulate rapid growth of algae and aquatic plants, producing an algal bloom. The dense algal layer blocks sunlight from reaching submerged plants, preventing them from photosynthesising and causing them to die. When the algae and dead plants decompose, bacteria consume large amounts of dissolved oxygen in the water. The resulting oxygen depletion (hypoxia) forces fish to suffocate and die. This sequence of events is known as eutrophication.
Examiner tips
- Use the exact terms: fertiliser runoff, nitrates/phosphates, algal bloom, light penetration, photosynthesis, bacterial respiration, hypoxia, eutrophication.
- Show the causal chain in order: runoff → nutrient enrichment → algal bloom → light block → plant death → bacterial decomposition → oxygen loss → fish death.
- Mention the key process (eutrophication) to demonstrate understanding.
Mark scheme (5 marks)
- Fertilisers (nitrates/phosphates) are washed into rivers/lakes by rain (leaching/run-off)
- Algae (or aquatic plants) use the extra nutrients and grow rapidly / algal bloom forms
- Algae block out light to plants below / light cannot penetrate the water
- Plants below the surface cannot photosynthesise and die / algae eventually die
- Bacteria decompose the dead organisms, using up oxygen in the water (by respiration), so fish suffocate / die due to lack of oxygen — this process is eutrophication
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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