Eutrophication
Exam-ready definition
The over-enrichment of water with nutrients (from fertiliser run-off or sewage), causing an algal bloom that blocks light, the death and decomposition of plants and algae, oxygen depletion by decomposer respiration, and the death of aquatic animals.
How this is marked
This is a sequencing question. The marks are for the correct order of the chain. The step students skip is the killer: it is the DECOMPOSERS respiring (using oxygen) that depletes the water, not the algae 'using up oxygen' directly.
In an exam answer
Explaining fish deaths: 'Fertiliser run-off causes an algal bloom; when the algae die, decomposers respire and use up the oxygen, so the fish die. This is eutrophication.'
Full topic guide: Organisms & their environment
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