Question

A lake near village suffered heavy mortality of fishes within a few days. Consider the following reasons for this.

(i) Lots of urea and phosphate fertiliser were used in the crops in the vicinity.

(ii) The area was sprayed with DDT by an aircraft.

(iii) The lake turned green and stinky.

(iv) Phytoplankton population 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. (i), (ii)
• B. (ii), (iii)
• C. (iii), (iv)
• D. (i), (iii)

Answer 1

Answer: (D)

(i), (iii)

The streams draining the urea and phosphate fertilisers into the lake introduced nutrients such as nitrogen and phosphorus, which encourage the growth of algae. As the lake's fertility increases, algal bloom occurs which is known as eutrophication in which the lake turns green and stinky. Algal bloom causes the deterioration of water quality and death of aquatic plants and animals like fishes.

Thus, the correct answer is option D.