Explain how the release of untreated sewage into rivers can reduce biodiversity in those rivers.

WJEC A-Level Biology (Wales) — 3.6 Human impact on the environment · Explain · 5 marks · View as Markdown

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

In some parts of the world, untreated sewage is discharged directly into rivers. This can have serious consequences for the organisms living in the river.

Model answer (5 marks)

The sewage contains large amounts of nutrients such as nitrates and minerals, which stimulate rapid growth of algae (an algal bloom). The dense algae cover the water surface and block sunlight from reaching submerged aquatic plants. Without light, these plants cannot photosynthesise and die. The dead plant and algal material is then decomposed by bacteria, which consume dissolved oxygen in the water. The resulting low oxygen (hypoxia) means fish and other aquatic animals cannot survive and die, thereby reducing the number of species present – i.e. biodiversity is lowered.

Examiner tips

  • Use the sequence: nutrients → algal bloom → light block → plant death → bacterial oxygen consumption → fish death
  • Mention hypoxia and link each step to biodiversity loss

Common mistakes

  • Forgetting to mention the role of bacteria in consuming oxygen
  • Describing algae as beneficial instead of harmful

Mark scheme (5 marks)

  1. Sewage contains large amounts of nutrients / nitrates / minerals that cause algae to grow rapidly (algal bloom)
  2. The algae block sunlight from reaching aquatic plants below the surface
  3. Aquatic plants die because they cannot photosynthesise
  4. Bacteria decompose the dead plants / algae and use up oxygen in the water
  5. Animals such as fish cannot survive without sufficient oxygen and die, reducing biodiversity

Key terms in this question

biodiversity · sewage

Related

More Human impact on the environment questions

▶ Try answering this question with AI marking (free) →