Eutrophication – IGCSE Chemistry Definition

Eutrophication is a 4–5 mark sequence question on Paper 4. The examiner gives one mark per step in the chain, so you must know the full sequence — not just “fertilisers cause pollution”. Every step is a separate marking point.

The 0620 definition

Eutrophication is the process by which excessive nutrients (particularly nitrates and phosphates) in a body of water lead to increased algal growth, oxygen depletion, and the death of aquatic organisms.

The eutrophication sequence (learn each step)

1. Excess fertiliser is washed / leached into a river or lake by rainwater
2. Nitrates and phosphates cause rapid growth of algae (an algal bloom)
3. The algae block sunlight from reaching underwater plants
4. Underwater plants die because they cannot photosynthesise
5. Bacteria decompose the dead plants and algae
6. The bacteria use up dissolved oxygen (aerobic decomposition)
7. Fish and other aquatic organisms die due to lack of oxygen

Each numbered point is a separate mark in the mark scheme. Skipping a step loses that mark.

Sources of excess nutrients

Source	Nutrient
Agricultural fertilisers	Nitrates, phosphates
Sewage	Nitrates, phosphates
Detergents	Phosphates
Animal waste / manure	Nitrates

Prevention

• Use fertilisers in appropriate amounts (not excess)
• Apply fertiliser at the right time (not before heavy rain)
• Create buffer zones of vegetation along waterways to absorb runoff
• Treat sewage to remove phosphates and nitrates before discharge

Worked exam question

A farmer applies excess fertiliser to fields near a lake. Describe the sequence of events that leads to the death of fish in the lake. [5]

Mark scheme

Fertiliser / nitrates / phosphates washed into the lake [1]; causes rapid growth of algae / algal bloom [1]; algae block light to underwater plants / plants die [1]; bacteria decompose dead organisms [1]; bacteria use up dissolved oxygen / oxygen levels fall, so fish die [1]

Common exam mistakes

• Saying “fertiliser kills the fish directly” — the fish are killed by oxygen depletion, not by the fertiliser itself. The full sequence must be described.
• Missing the role of bacteria — it is bacterial decomposition that uses up oxygen, not the algae themselves.
• Forgetting to mention that light is blocked — this is why the underwater plants die, not because the fertiliser poisons them.