In a village, farmers started cultivating crops all around a lake which was always filled with water. They added lot of materials W to the soil in their fields to increase the yield of crops. Soon they found that the whole surface of water in the lake was covered with a green layer made up of tiny organisms X. After some time, the fish present in the lake started dying in large numbers because they could not get sufficient Y due to a process Z which had occurred in lake water and drained out all the Y present in the water of lake.
(a) What do you think materials W are ?
(b) Name (i) X, and (ii) Y.
(c) What is the process Z known as?
(d) What happens during the process Z which uses up all the Y present in lake water?
(e) What does this example tell us about the use of materials like W?
(a) W should be fertilizers. They are substances which help increase the yield of the crop.
(b) (i) The green layer of tiny organisms indicates the presence of algae. So X is algae.
(ii) The substance absence of which made the fish in lake to die is oxygen.
(c) The process Z is known as Eutrophication.
(d) In eutrophication, excessive richness of nutrients in a lake causes a dense growth of algae. When these algae die, bacteria use up all the oxygen dissolved in the water to decompose the dead algae.
(e) This process indicates that excessive use of fertilisers should not be done in fields as ot harms the environment.