4. Write critical notes on the following:
(a) Eutrophication
Eutrophication- The term eutrophication is used to refer to the excessive growth of algae, plants and animals in water bodies due to the nutrient enrichment of the water body with nitrogen and phosphorous. Eutrophication can be natural or accelerated. In the case of natural eutrophication, natural ageing causes nutrient enrichment of the water body. This is a slow process that is not detectable in a human lifetime. A young water body has clear, cold water with no nutrient enrichment. The nutrients are added via runoff and streams draining into it. Gradually the water becomes shallower and warmer. Due to this, marsh plants grow in it, floating plants appear. The waterbody is gradually filled upon the shores and changed into the land mass. In cultural or accelerated eutrophication, nutrient enrichment is due to human activities like the passage of sewage, industrial effluents and run-off rich nitrated and phosphates from fertilised fields. These nutrients cause dense growth of plants and planktonic algae over the water body.