# Explain how the release of untreated sewage into rivers can reduce biodiversity in those rivers.

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

> In some parts of the world, untreated sewage is discharged directly into rivers. This can have serious consequences for the organisms living in the river.

## Mark scheme (5 marks)

1. Sewage contains large amounts of nutrients / nitrates / minerals that cause algae to grow rapidly (algal bloom)
2. The algae block sunlight from reaching aquatic plants below the surface
3. Aquatic plants die because they cannot photosynthesise
4. Bacteria decompose the dead plants / algae and use up oxygen in the water
5. Animals such as fish cannot survive without sufficient oxygen and die, reducing biodiversity

## Key terms

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

## 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-release-of-untreated-b0911151) · Published by Druglandscape Ltd.