Describe how the overuse of fertilisers on farmland can harm aquatic ecosystems. Include in your answer the name of the process involved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Farmers sometimes apply more fertiliser to their fields than crops can absorb. Rain can wash excess fertiliser into nearby rivers and lakes.
Model answer (4 marks)
Excess fertiliser (nitrates or phosphates) is washed off the fields by rain (leaching/run‑off) into rivers or lakes. The added nutrients stimulate rapid growth of algae or other water plants, forming an algal bloom on the surface. The dense algal layer blocks sunlight from reaching submerged plants, causing them to die. When the algae decompose, the organisms that break them down consume large amounts of dissolved oxygen, leaving the water oxygen‑depleted. Fish and other aquatic organisms then die from the lack of oxygen – a process called eutrophication.
Examiner tips
- Use the exact terms: leaching/run‑off, algae/algal bloom, eutrophication
- Show the sequence: fertiliser → runoff → nutrient enrichment → algal bloom → light block/death of plants → oxygen depletion → fish death
- Mention the key process name (eutrophication)
- Keep answer concise, 4‑point structure
Common mistakes
- Confusing eutrophication with pollution in general
- Omitting the word ‘run‑off’ or ‘leaching’
- Failing to state that the process is called eutrophication
Mark scheme (4 marks)
- Excess fertiliser (nitrates/phosphates) is washed into rivers or lakes by rain (leaching/run-off)
- Algae (or water plants) grow rapidly / algal bloom forms on the water surface
- Algae block sunlight reaching plants below, causing them to die / decomposing organisms use up oxygen
- Fish and other aquatic organisms die due to lack of oxygen; this process is called eutrophication
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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