Explain how pollution of a river with fertilisers from farmland reduces biodiversity in the river ecosystem.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Farmers sometimes apply fertilisers to fields near rivers. Rain can wash these fertilisers into the river.
Model answer (4 marks)
Fertiliser runoff increases the concentration of nutrients (especially nitrogen and phosphorus) in the river, stimulating excessive growth of algae and aquatic plants on the water surface (algal bloom). The dense algal layer blocks sunlight from reaching submerged plants, causing them to die. The dead plant material is then broken down by microorganisms and decomposers, which consume dissolved oxygen in the water. The resulting low‑oxygen (hypoxic) conditions prevent fish and invertebrates from surviving, leading to their death and a reduction in overall biodiversity.
Examiner tips
- Use the sequence: fertiliser → algal bloom → light block → plant death → oxygen depletion → animal death
- Include key terms: nutrients, algae, hypoxia, biodiversity
Common mistakes
- Failing to mention the role of nutrients or light blockage
- Using vague terms like "pollution" instead of specific effects
- Not linking oxygen depletion to loss of fish and invertebrates
Mark scheme (4 marks)
- Fertilisers cause increased growth of algae / plants on the water surface (algal bloom)
- Algae block light from reaching plants below the surface, so underwater plants die
- Microorganisms / decomposers break down the dead plant material, using up oxygen in the water
- Animals (e.g. fish / invertebrates) cannot survive in low-oxygen water and die, reducing biodiversity
Key terms in this question
Related
- Revision notes: Biodiversity and the effect of human interaction on ecosystems →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →