# Describe how the overuse of fertilisers on farmland can harm aquatic ecosystems. Include in your answer the name of the process involved.

> Cambridge International IGCSE Chemistry (0620) — 10.2 Fertilisers · Describe · 4 marks

> Farmers sometimes apply more fertiliser to their fields than crops can absorb. Rain can wash excess fertiliser into nearby rivers and lakes.

## Mark scheme (4 marks)

1. Excess fertiliser (nitrates/phosphates) is washed into rivers or lakes by rain (leaching/run-off)
2. Algae (or water plants) grow rapidly / algal bloom forms on the water surface
3. Algae block sunlight reaching plants below, causing them to die / decomposing organisms use up oxygen
4. Fish and other aquatic organisms die due to lack of oxygen; this process is called eutrophication

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/describe-how-the-overuse-of-fertilisers-f8fd9dd2) · Published by Druglandscape Ltd.