Untreated sewage is sometimes discharged into rivers. Explain how the discharge of untreated sewage into a river can lead to the death of fish.

Cambridge International IGCSE Biology (0610) — 20.3 Pollution · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Untreated sewage contains excess nutrients (nitrogen, phosphorus) that stimulate rapid growth of algae on the water surface.
Algal blooms form a thick layer that blocks sunlight, preventing submerged plants from photosynthesising.
When the plants die they are decomposed by bacteria and other microorganisms, which consume dissolved oxygen in the water.
The oxygen level falls below the requirement for fish, leading to their death.

Examiner tips

  • Use the word "stimulate" or "increase" for nutrient effect; mention "algal bloom". Show the chain: nutrients → algae → light block → plant death → bacterial respiration → low O₂ → fish death. Use UK spelling and correct terminology (oxygen, photosynthesis).

Common mistakes

  • Confusing sewage as a source of oxygen instead of nutrients. Omitting the step that bacteria consume oxygen during decomposition. Using vague terms like "pollution" without linking to specific effects on fish.

Mark scheme (4 marks)

  1. Untreated sewage provides nutrients / mineral salts that cause increased growth of algae (algal bloom) on the water surface.
  2. The algae blocks / prevents sunlight from reaching plants below the surface, so the plants cannot photosynthesise.
  3. Dead plants are broken down by decomposers / microorganisms, which use up oxygen from the water.
  4. The reduced oxygen concentration in the water causes fish to die.

Key terms in this question

untreated sewage

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