# Explain how the release of raw sewage into a river can lead to a decrease in the number of fish living in that river.

> Eduqas GCSE Biology — 1.6.3 Human impact on the environment · Explain · 4 marks

## Mark scheme (4 marks)

1. Raw sewage contains large amounts of nutrients / nitrates / minerals
2. This causes rapid growth / multiplication of algae / microorganisms on the water surface (algal bloom)
3. The algae block light reaching underwater plants, so underwater plants cannot photosynthesise and die
4. Decomposers / bacteria break down the dead plants and use up oxygen, so fish cannot respire and die

## Key terms

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

## Related

- [Revision notes for Eduqas GCSE Biology](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-release-of-raw-9499ef34) · Published by Druglandscape Ltd.