Explain how the dumping of untreated sewage into a river can lead to a decrease in the number of fish in that river.

WJEC GCSE Biology (Wales) — 1.6.3 Human impact on the environment · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In some countries, untreated sewage is discharged directly into rivers. This can have serious consequences for the organisms that live there.

Model answer (4 marks)

1. Sewage contains excess nutrients (nitrogen, phosphorus) that stimulate rapid growth of algae and bacteria.
2. The algal bloom forms a dense surface layer that blocks sunlight from reaching submerged aquatic plants.
3. When the algae and other organic matter die, bacteria decompose them, consuming dissolved oxygen in the process.
4. The resulting low oxygen (hypoxic) conditions prevent fish from respiring, leading to mass mortality and a reduced fish population.

Examiner tips

  • Use the word "stimulate" or "promote" for nutrient effect; mention "hypoxia" for low oxygen. Show the causal chain: nutrients → algal bloom → light blockage → decomposition → oxygen depletion → fish death. Use UK spelling (e.g., "decompose").

Common mistakes

  • Confusing sewage as a source of oxygen rather than a source of nutrients. Failing to mention the blocking of light or the role of decomposing bacteria. Using vague terms like "pollution" without linking to oxygen depletion.

Mark scheme (4 marks)

  1. Sewage provides nutrients / minerals that cause increased growth of algae / bacteria / microorganisms
  2. Algae / plants on the surface block light from reaching plants / pondweed below
  3. Dead organic matter / dead plants are decomposed by bacteria / microorganisms which use up oxygen in the water
  4. Fish cannot respire / do not have enough oxygen to survive and die

Key terms in this question

sewage

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