Explain how the release of raw sewage into a river can lead to a decrease in the number of fish living in that river.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Raw sewage contains large amounts of nutrients such as nitrates and minerals.
This stimulates rapid growth of algae and microorganisms on the water surface, forming an algal bloom.
The dense algal layer blocks light from reaching submerged plants, preventing photosynthesis and causing the plants to die.
Decomposers and bacteria then break down the dead plant material, consuming oxygen in the process; the resulting low dissolved oxygen levels mean fish cannot respire and die.
This stimulates rapid growth of algae and microorganisms on the water surface, forming an algal bloom.
The dense algal layer blocks light from reaching submerged plants, preventing photosynthesis and causing the plants to die.
Decomposers and bacteria then break down the dead plant material, consuming oxygen in the process; the resulting low dissolved oxygen levels mean fish cannot respire and die.
Examiner tips
- Use the exact sequence: nutrients → algal bloom → light block → plant death → oxygen depletion → fish death.
- Show the causal chain clearly; each step earns a mark. Use terms like 'dissolved oxygen' and 'photosynthesis'.
Common mistakes
- Failing to mention the role of dissolved oxygen or photosynthesis.
- Using vague terms like 'pollution' instead of specific nutrients or algal bloom.
Mark scheme (4 marks)
- Raw sewage contains large amounts of nutrients / nitrates / minerals
- This causes rapid growth / multiplication of algae / microorganisms on the water surface (algal bloom)
- The algae block light reaching underwater plants, so underwater plants cannot photosynthesise and die
- Decomposers / bacteria break down the dead plants and use up oxygen, so fish cannot respire and die
Key terms in this question
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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