Eutrophication is a process that can reduce biodiversity in a freshwater lake. Explain how the use of fertilisers on nearby farmland leads to eutrophication and results in the death of animal species living in the lake.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Farmers often apply nitrate and phosphate fertilisers to their fields to increase crop yields. After heavy rainfall, some of these minerals are washed from the soil into a nearby freshwater lake.
Model answer (4 marks)
Fertiliser runoff from the fields carries excess nitrate and phosphate into the lake, which act as nutrients for algae.
1. The added nutrients stimulate rapid algal growth at the water surface.
2. The dense algal layer blocks light from reaching submerged aquatic plants.
3. Without light the plants cannot photosynthesise, so they die.
4. Decomposers break down the dead plant matter, consuming oxygen and leaving the water oxygen‑poor, causing fish and other animals to suffocate and die.
1. The added nutrients stimulate rapid algal growth at the water surface.
2. The dense algal layer blocks light from reaching submerged aquatic plants.
3. Without light the plants cannot photosynthesise, so they die.
4. Decomposers break down the dead plant matter, consuming oxygen and leaving the water oxygen‑poor, causing fish and other animals to suffocate and die.
Examiner tips
- Use the sequence: fertiliser → algae → light block → plant death → oxygen depletion → animal death.
- Include the key terms: leached, excessive growth, photosynthesis, decomposers, oxygen depletion.
- Show the causal chain clearly to hit all four marks.
Common mistakes
- Failing to mention the blocking of light or the role of decomposers.
- Using vague language such as "more algae" without explaining the nutrient source.
- Mixing up the order of events or omitting the final oxygen‑depletion step.
Mark scheme (4 marks)
- Fertilisers are leached/washed from the land into the lake, causing excessive growth of algae at the water surface.
- The algae at the surface block/prevent light from reaching aquatic plants below.
- Aquatic plants can no longer photosynthesise and die.
- Decomposers break down the dead plant material and use up oxygen, reducing the amount of oxygen available for animal species, which die.
Key terms in this question
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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