When more nitrate fertiliser is used on farmland than the crops can take up, the excess can be washed off the land into nearby lakes and rivers, where it se...
Question 1 Report
When more nitrate fertiliser is used on farmland than the crops can take up, the excess can be washed off the land into nearby lakes and rivers, where it sets off a series of changes in the water. This sequence of changes, which can end with the death of fish, is known as eutrophication. A student was given a flow diagram that sets out the stages of this process in order. The flow diagram summarises the stages of eutrophication that follow when excess nitrate fertiliser enters a lake, but two of the stages have been left blank.
(a) Explain why plants need nitrate ions from the soil for healthy growth. [2]
(b) Using the flow diagram, explain fully how excess nitrate fertiliser leads to the death of fish in the lake. [6]
(c) Explain why the number of bacteria in the water rises sharply after an algal bloom dies. [2]
(d) Suggest two ways a farmer could reduce the amount of nitrate reaching the lake. [2]
Answer Details
This question tests eutrophication: how excess nitrate fertiliser can kill fish in a lake.
(a) Why plants need nitrate. [2] Nitrate supplies a source of nitrogen [1], which the plant needs to make amino acids and so proteins (accept DNA or chlorophyll) for growth [1].
(b) How excess nitrate kills the fish. [6] Follow the flow diagram as a chain, one mark per step (max 6):
- the extra nitrate causes rapid growth of algae, an algal bloom [1];
- the algae cover the surface and block out light [1];
- plants and algae below cannot photosynthesise and die [1];
- the dead matter is decomposed by bacteria [1];
- the bacteria multiply and respire aerobically [1];
- they use up the dissolved oxygen, so the fish cannot respire and die [1].
(c) Why bacteria numbers rise after the bloom dies. [2] The dead algae provide plenty of food for the decomposer bacteria [1], so the bacteria reproduce/multiply rapidly [1].
(d) Two ways to reduce nitrate reaching the lake. [2] Any two: apply less fertiliser, or only when crops are actively growing [1]; avoid spreading before heavy rain, leave buffer strips, or use organic manure [1]. Exam tip: the killer step to make explicit is that the oxygen is removed by respiring decomposers, not by the algae themselves.
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