# 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

> Farmers apply nitrate fertilisers to their fields to increase crop yields. Rain can wash these fertilisers from the soil into nearby bodies of water.

## 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

- [Revision notes for Pearson Edexcel International GCSE Biology (4BI1)](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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