Describe the process of eutrophication and explain how it leads to a reduction in the number of animal species living in a lake.

Pearson Edexcel International GCSE Biology (4BI1) — 4.4 Human influences on the environment · Describe and Explain · 4 marks · View as Markdown

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

Farmers often apply nitrate fertilisers to fields surrounding lakes. When it rains, these fertilisers can enter the lake water.

Model answer (4 marks)

Fertiliser (nitrates) enter the lake, causing excessive growth of algae (an algal bloom) at the water surface.
The algae block or prevent light from reaching aquatic plants below the surface.
The aquatic plants die; decomposers/bacteria break down the dead plants (and algae), using up oxygen in the water.
Reduced oxygen levels mean animals (e.g. fish) cannot respire and die, so the number of animal species decreases.

Examiner tips

Use the exact sequence: fertiliser → algal bloom → light blockage → plant death → decomposition → oxygen depletion → animal death.
Show cause–effect links clearly; mention oxygen (O₂) depletion and fish mortality.
Include the term "eutrophication" in the opening sentence to demonstrate understanding.

Common mistakes

Writing that fertiliser causes *more* oxygen, rather than depletion.
Failing to mention the blocking of light or the role of decomposers.
Using vague terms like "pollution" instead of specifying nitrates and algal bloom.

Mark scheme (4 marks)

Fertiliser (nitrates) enter the lake, causing excessive/rapid growth of algae (algal bloom) at the water surface
The algae block/prevent light from reaching aquatic plants below the surface
Aquatic plants die; decomposers/bacteria break down the dead plants (and algae), using up oxygen in the water
Reduced oxygen levels mean animals (e.g. fish) cannot respire and die / the number of animal species decreases

Key terms in this question

eutrophication