A farmer spreads nitrate fertiliser on a field next to a lake. After heavy rain, many fish in the lake die.
Explain how the fertiliser could lead to the death of the fish. [6]
- fertiliser / nitrates washed / leached into the lake (by the rain)(1)
- rapid growth of algae (algal bloom) on the surface; allow nitrates used by algae / plants for growth / to make proteins(1)
- algae block light so plants (below the surface) cannot photosynthesise and die; allow algae / plants die(1)
- (dead algae and plants) are decomposed by bacteria / decomposers; allow number of decomposers / bacteria increases(1)
- (decomposers) respire aerobically, using up oxygen (in the water); allow oxygen concentration in the water falls(1)
- fish die from lack of oxygen / cannot respire aerobically; reject fish die from the fertiliser being toxic(1)
- Answer: Nitrates leach into lake → algal bloom → light blocked, plants die → decomposers respire using up oxygen → fish die.
- Examiner tip: The oxygen is used up by the DECOMPOSERS respiring — not by the algae — this is the key mark many miss.