# Explain how pollution of a river with fertilisers from farmland reduces biodiversity in the river ecosystem.

> AQA GCSE Biology (8461) — 4.7.3 Biodiversity and the effect of human interaction on ecosystems · Explain · 4 marks

> Farmers sometimes apply fertilisers to fields near rivers. Rain can wash these fertilisers into the river.

## Mark scheme (4 marks)

1. Fertilisers cause increased growth of algae / plants on the water surface (algal bloom)
2. Algae block light from reaching plants below the surface, so underwater plants die
3. Microorganisms / decomposers break down the dead plant material, using up oxygen in the water
4. Animals (e.g. fish / invertebrates) cannot survive in low-oxygen water and die, reducing biodiversity

## Key terms

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

## Related

- [Revision notes for AQA GCSE Biology (8461)](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-pollution-of-a-river-f069003a) · Published by Druglandscape Ltd.