# Explain how the dumping of untreated sewage into a river can lead to a decrease in the number of fish in that river.

> WJEC GCSE Biology (Wales) — 1.6.3 Human impact on the environment · Explain · 4 marks

> In some countries, untreated sewage is discharged directly into rivers. This can have serious consequences for the organisms that live there.

## Mark scheme (4 marks)

1. Sewage provides nutrients / minerals that cause increased growth of algae / bacteria / microorganisms
2. Algae / plants on the surface block light from reaching plants / pondweed below
3. Dead organic matter / dead plants are decomposed by bacteria / microorganisms which use up oxygen in the water
4. Fish cannot respire / do not have enough oxygen to survive and die

## Key terms

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

## Related

- [Revision notes for WJEC GCSE 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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