CBSE Class 10 Science 2019 · Paper 31/3/2 · Question 12

Reason for the excessive growth of plants: the fertilisers contain plant nutrients such as nitrates and phosphates. Rain and irrigation water drain the excess fertiliser from the surrounding fields into the lake. In this nutrient-rich water the green floating plants (algae and other water plants) grow and multiply very fast and cover the lake. This is called eutrophication. Reason for the death of aquatic animals: the layer of floating plants blocks sunlight, so the plants under the surface cannot photosynthesise and die, and no oxygen is added to the water. The floating plants use up the dissolved oxygen in respiration, and bacteria decomposing the dead plants use up still more. With so little dissolved oxygen left, the fish and other aquatic animals cannot respire and die in large numbers.