Question

Read the following statements :-
A lake near a village suffered heavy morality of fishes within a few days. Consider the following reasons for this;
(i)Lots of urea and phosphate fertilizer were used in the crops in the vicinity
(ii)The area was sprayed with DDT by an aircraft
(iii)The lake water turned green and stinky
(iv)Phytoplankton populations in the lake declined initially thereby greatly reducing photosynthesis
Which two of the above were the main causes of fish mortality in the lake ?

• A. (iii) and (iv)
• B. (ii) and (iii)
• C. (i) and (ii)
• D. (i) and (iii)

Answer 1

The correct option is D (i) and (iii)

The heavy mortality of fishes in the lake within a span of few days is due to Eutrophication.

Eutrophication is the phenomenon of nutrient enrichment of nitrogen and phosphorus in a water body that supports a dense growth of plants and animal life.

Due to nutrient enrichment, excess growth of algae occurs which is called algal bloom. When these algae die, their organic matter gets decomposed, and this results in the depletion of oxygen leading to death of aquatic animals. Hence, this indicates that lot of urea and phosphate fertilizers are were used in nearby crops. The lake water could have turned green and stinky due to excess growth of Algae.