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Sewage, fertiliser and eutrophication

Human influences on ecosystems · Pollution · note 1 of 3

Spec 20.3.1, 20.3.4Download all Pollution notes (PDF)Download all notes (PDF)5 pages

Sewage, fertiliser and eutrophicationSpec 20.3.1, 20.3.4

In short

Eutrophication is the chain of events when extra nitrate and other ions from excess fertiliser or untreated sewage enter water. Algae and water plants grow fast and block light, so producers deeper down die. Decomposers increase and their aerobic respiration uses up dissolved oxygen, so fish and other organisms that need dissolved oxygen die.

Untreated sewage and excess fertiliser both contain nitrates and other ions. If they get into rivers and lakes, they harm aquatic ecosystems.

  • Untreated sewage also contains microorganisms that cause disease and organic matter that decomposers break down using oxygen.
  • Excess fertiliser is washed (leached) from fields by rain into streams and rivers.
  • The extra ions cause water plants and algae to grow quickly. When they die, decomposers break them down and use up oxygen in the water, so fish and other aquatic organisms die.
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

Eutrophication

Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
  1. Nitrate and other ions increase in availability in the water.
  2. Producers (algae and water plants) grow faster, forming an algal bloom that blocks light.
  3. Producers deeper in the water cannot photosynthesise and die; there is increased decomposition after their death.
  4. Decomposers (bacteria) increase in number and carry out increased aerobic respiration.
  5. The decomposers use up oxygen, so there is a reduction in dissolved oxygen.
  6. Organisms that require dissolved oxygen, such as fish and insect larvae, die.
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
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. (opens full size in a new tab)
Eutrophication: from fertiliser run-off to the death of fish.
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
Exam tip:

Do not say the algae 'use up the oxygen'. It is the decomposers' aerobic respiration, after the producers die, that lowers dissolved oxygen.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

What is eutrophication and what causes it?

Supplement Eutrophication is caused by extra nitrate and other ions entering water from excess fertiliser or untreated sewage. Algae and water plants grow faster, forming an algal bloom that blocks light. Producers deeper in the water die, and decomposers increase. Their aerobic respiration uses up dissolved oxygen, so fish and other organisms that need oxygen die.

How does plastic pollution affect marine life?

Plastic pollution harms marine life because plastics are non-biodegradable, so they last a very long time. Turtles, seabirds and fish swallow plastic, which can block their digestive system so they starve. Animals become tangled in plastic and drown or cannot move. Plastic also breaks into tiny pieces that enter food chains.

What causes the enhanced greenhouse effect?

The enhanced greenhouse effect is caused by human activities increasing the concentrations of carbon dioxide and methane in the atmosphere. Burning fossil fuels and forests releases carbon dioxide, while cattle, rice paddy fields and landfill sites release methane. These gases trap more infrared radiation, so the Earth's average temperature rises, causing climate change.

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