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 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 ?

Answer

(a) Fertilisers (b) (i) Algae (ii) Oxygen (c ) Eutrophication (d) When algae die, bacteria use all the oxygen dissovled in water to decompose dead algae (e) Excessive use of fertilisers in fields is not good for the environment