Eutrophication — Human Impact on the Environment | IGCSE Biology

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 10% of your exam marks.

increasing

Medium

Increasing10%

Climate change, deforestation, and biodiversity loss are growing in frequency as contemporary issues.

Eutrophication is the over-enrichment of a body of water with nutrients, leading to the collapse of its ecosystem. It is the leading cause of dead zones in lakes, rivers and coastal seas worldwide.

The five-step sequence

Excess nutrients (nitrate and phosphate) enter a body of water. Most commonly from agricultural fertilisers washed off fields by rain, or from sewage containing detergents.
The extra nitrates and phosphates cause explosive growth of algae at the water surface (an algal bloom).
The thick algal layer blocks sunlight from reaching the deeper-water plants. Those plants cannot photosynthesise and die.
The dead plants and (later) the algae are broken down by decomposer bacteria. The decomposers multiply rapidly and use up the dissolved oxygen in the water as they respire.
Fish, invertebrates and other aquatic animals suffocate because there is no longer enough oxygen in the water. The water body becomes a "dead zone".

Exam tip

Describe the sequence of events that leads to fish dying after fertiliser runoff

What comes up: questions ask you to describe or explain eutrophication, usually for 4–8 marks in the form of a step-by-step chain.

Write (five marks): (1) fertiliser/nitrates (and phosphates) wash off fields into the water body; (2) algae grow rapidly at the surface, forming an algal bloom that blocks light from reaching plants below; (3) the underwater plants die because they cannot photosynthesise; (4) decomposer bacteria break down the dead organic matter and their numbers increase; (5) the bacteria use up dissolved oxygen during respiration, so aquatic animals suffocate and die.

Watch out: it is the decomposer bacteria (through their respiration) that deplete the oxygen and kill the fish, not the algae directly. The mark scheme credits "bacteria/microorganisms respire" specifically for the oxygen-depletion step — writing "algae use up oxygen" will not score that mark.

Why fertilisers are the main culprit

Modern agriculture uses huge amounts of nitrogen-based fertiliser to maximise crop yields. Plants only absorb a fraction of the applied nitrate; the rest stays in the soil and is leached out by rainfall into rivers and groundwater. Even small fields can release enough nitrate to disturb a downstream lake.

Phosphates from older washing detergents used to make the problem worse; phosphate-free detergents have largely solved this part of the problem in the UK, but agricultural runoff remains the dominant source.

Where eutrophication happens

Lakes and slow rivers in farming areas
Coastal seas at the mouths of large rivers draining intensive agricultural land. The Gulf of Mexico has a dead zone the size of New Jersey every summer, fed by nitrate runoff from the Mississippi River
The Baltic Sea, surrounded by farming countries, has experienced widespread eutrophication

Solutions

Reduce fertiliser use by applying only as much as the crop will absorb, at the right time of year
Buffer strips: leave strips of grass and trees along rivers and ditches to absorb runoff before it reaches the water
Improve sewage treatment to remove nitrate and phosphate before discharging treated water
(now standard in the UK)