In a village in Karnataka,people started cultivating crops all around a lake which was always filled with water. They added fertilisers to their fields in order to enhance the yield. Soon they discovered that the waterbody was completely covered with green floating plants and fishes started dying in large numbers. Analyse the situation and give reasons for the excessive growth of plants and the death of fish in the lake.

(N/A) The excessive use of fertilisers in the fields led to the runoff of nutrients into the lake during rains.
These fertilisers are rich in phosphates and nitrates,which act as nutrients for aquatic plants,leading to their rapid and excessive growth,a process known as $Eutrophication$.
As the surface of the water became completely covered with these plants,it blocked sunlight from reaching deeper layers.
Furthermore,the decomposition of these plants by microorganisms consumed a large amount of dissolved oxygen,leading to its depletion.
This lack of dissolved oxygen and the change in the ecosystem resulted in the mass death of fish.