# Untreated sewage is sometimes discharged into rivers. Explain how the discharge of untreated sewage into a river can lead to the death of fish.

> Cambridge International IGCSE Biology (0610) — 20.3 Pollution · Explain · 4 marks

## Mark scheme (4 marks)

1. Untreated sewage provides nutrients / mineral salts that cause increased growth of algae (algal bloom) on the water surface.
2. The algae blocks / prevents sunlight from reaching plants below the surface, so the plants cannot photosynthesise.
3. Dead plants are broken down by decomposers / microorganisms, which use up oxygen from the water.
4. The reduced oxygen concentration in the water causes fish to die.

## Key terms

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

## Related

- [Revision notes for Cambridge International IGCSE Biology (0610)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/untreated-sewage-is-sometimes-discharged-into-8c46d04a) · Published by Druglandscape Ltd.