Question

Eutrophication of water bodies leading to the killing of fishes mainly due to non-availability of
a.Food
b.Light
c.Essential minerals
d.Oxygen

Answer 1

Hint: Eutrophication is the natural ageing of a lake by nutrient enrichment of its water. The streams and the lake contain nutrients like nitrogen and phosphorus which increase the growth of aquatic organisms.

Complete answer:
Eutrophication is the process by which the accumulation of silt and organic debris takes place at the lakes or water streams. Due to this the fertility of the lake increases and as a result, the life of the aquatic plants and animals become dangerous while the organic matter starts to deposit at the bottom of the lake. The marshy plants take root in the shallows and begin to fill up the original lake basin. The presence of large amounts of nutrients in water leads to a growth of planktonic algae and it is known as algal bloom which gives a different colour to the water bodies. Algal bloom causes a decline in water quality and fish mortality.
As a result of eutrophication the concentration of oxygen fall and due to this there is a death of aquatic animals due to suffocation. The algal bloom is very toxic for animals and humans. The lakes in different parts of the world have been severely eutrophied by sewage, agricultural and industrial wastes.
Hence, the correct answer is the non-availability of oxygen.

Note: The main source of nitrogen pollutants is a run-off from the agricultural lands while the most phosphorus pollutants come from the household sewage. Due to the overgrowth of algae in the lake caused biological oxygen demand.