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

> WJEC A-Level Biology (Wales) — 3.6 Human impact on the environment · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Fertilisers (nitrates/phosphates) are washed into rivers/lakes by rain (leaching/run-off)
2. Algae (or aquatic plants) use the extra nutrients and grow rapidly / algal bloom forms
3. Algae block out light to plants below / light cannot penetrate the water
4. Plants below the surface cannot photosynthesise and die / algae eventually die
5. 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

- [fertilisers](https://www.gradenine.co.uk/glossary/fertilisers)

## Related

- [Revision notes for WJEC A-Level Biology (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-use-of-fertilisers-bc6a250a) · Published by Druglandscape Ltd.