Eutrophication

Ecology and the environment · Human influences on the environment · note 5 of 5

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EutrophicationSpec 4.17

In short

Eutrophication is when too many mineral ions, such as nitrates and phosphates from fertilisers, are leached into lakes and rivers. Algae grow quickly, forming an algal bloom that blocks light, so plants below cannot photosynthesise and die. Decomposing bacteria feed on the dead material and use up the oxygen in aerobic respiration, so fish die.

Eutrophication happens when too many mineral ions, such as nitrates and phosphates from fertilisers, enter lakes and rivers. Leaching is the washing of dissolved minerals out of the soil by rain. Fertiliser that crops have not taken up is leached into streams and rivers.

Nitrates and other minerals from fertilisers are leached from fields into the water.
Algae in the water grow very quickly because of the extra minerals, forming a layer on the surface (an algal bloom).
The layer blocks light, so plants and algae lower down cannot photosynthesise and die.
Bacteria (decomposers) feed on the dead material and their numbers increase.
The bacteria use up oxygen in the water for aerobic respiration.
Fish and other aquatic animals die from lack of oxygen, so biodiversity falls.

Common mistake:

The fish do not die because the algae poison them. The main cause is the decomposers feeding on the dead plants and algae, which use up the oxygen.

Lake cross-section showing fertiliser run-off, an algal bloom blocking light, dead plants on the lake bed, bacteria using up oxygen and a dead fish, numbered to match a six-step flow chart of eutrophication. — Eutrophication: extra nitrates cause an algal bloom, plants die, decomposers use up the oxygen and fish die.

Quick check

What gas causes acid rain?
Show answer
Sulfur dioxide.
Why is carbon monoxide poisonous?
Show answer
It combines with haemoglobin, so the blood carries less oxygen.
Name the five greenhouse gases in the spec.
Show answer
Water vapour, carbon dioxide, nitrous oxide, methane and CFCs.
Why does sewage pollution kill fish?
Show answer
Decomposers feeding on it use up the oxygen in the water.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

What is eutrophication and what causes it?

Eutrophication happens when too many mineral ions, such as nitrates and phosphates from fertilisers, are leached into lakes and rivers. Algae grow rapidly and block light, so plants below die. Decomposing bacteria feed on the dead material and use up the oxygen in aerobic respiration, so fish and other animals die.

How does the enhanced greenhouse effect cause global warming?

The Earth gives out infrared radiation, and greenhouse gases absorb some of it and radiate it back towards the Earth. Human activities, such as burning fossil fuels, increase these gases, so more infrared radiation is trapped. The average surface temperature rises, which is global warming, causing rising sea levels and more extreme weather.

How does carbon monoxide affect the body?

Carbon monoxide combines with haemoglobin in red blood cells much more strongly than oxygen does. Haemoglobin combined with carbon monoxide cannot carry oxygen, so less oxygen reaches the tissues and cells cannot respire aerobically enough. This causes headaches, dizziness and fainting, and a high dose can cause death.

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