Farmers add fertilisers to their fields. Fertilisers help crops to grow. They hold nitrates and other minerals. Plants need these to grow. When it rains, so...
Question 1 Report
Farmers add fertilisers to their fields. Fertilisers help crops to grow. They hold nitrates and other minerals. Plants need these to grow. When it rains, some fertiliser is washed away. It runs into rivers and lakes. The extra nitrate makes algae grow fast. The algae form a thick green layer on the water. This causes big problems for other living things. In some lakes many fish and animals die. This whole process is called eutrophication. A student was asked to explain the steps. The steps go from fertiliser entering a lake to the death of the fish.
(a) Name the substance in fertiliser that causes the algae to grow quickly. [1]
(b) Explain why the rapid growth of algae over the surface of the water causes the water plants below to die. [3]
(c) Explain how the death of the water plants and algae leads to a fall in the amount of oxygen in the water. [3]
(d) Explain why the fall in the amount of dissolved oxygen causes the fish to die. [2]
(e) Suggest two ways in which a farmer could reduce this type of water pollution. [2]
Answer Details
This question tests the sequence of eutrophication, from fertiliser reaching a lake to the death of the fish. The graph shows dissolved oxygen dropping just downstream of the nutrient input before recovering.
(a) The substance in fertiliser that makes the algae grow quickly is nitrate [1] (accept phosphate).
(b) Rapid algal growth kills the water plants below because the algae form a thick layer over the surface of the water [1]; this blocks the light from reaching the plants below [1]; so the plants cannot photosynthesise and they die [1].
(c) The dead plants and algae lower the oxygen because they are broken down by decomposers / bacteria [1]; the number of these bacteria increases greatly [1]; and they use up the dissolved oxygen in the water for their own respiration [1].
(d) The fall in dissolved oxygen kills the fish because there is not enough dissolved oxygen for the fish to respire [1]; so the fish suffocate and die [1].
(e) Any two ways a farmer could reduce this pollution (1 each): use less fertiliser; do not spread fertiliser near rivers; do not spread fertiliser just before heavy rain; or leave a buffer strip of plants along the river to absorb run-off [2].
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