# Describe the process of eutrophication and explain how it leads to a reduction in the number of animal species living in a lake.

> Pearson Edexcel International GCSE Biology (4BI1) — 4.4 Human influences on the environment · Describe and Explain · 4 marks

> Farmers often apply nitrate fertilisers to fields surrounding lakes. When it rains, these fertilisers can enter the lake water.

## Mark scheme (4 marks)

1. Fertiliser (nitrates) enter the lake, causing excessive/rapid growth of algae (algal bloom) at the water surface
2. The algae block/prevent light from reaching aquatic plants below the surface
3. Aquatic plants die; decomposers/bacteria break down the dead plants (and algae), using up oxygen in the water
4. Reduced oxygen levels mean animals (e.g. fish) cannot respire and die / the number of animal species decreases

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Biology (4BI1)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/describe-the-process-of-eutrophication-and-12bf6c6b) · Published by Druglandscape Ltd.