A lake near a village suffered heavy mortality of fishes within a few days. Which of the following statements could be the correct explanation for this ?
(i) Lots of urea and phosphate fertillisers were used in the crops in the nearby fields.
(ii) The croplands of the village were fertillised fields rich in nitrate and phosphate.
(iii) The lake water turned green and stinky.
(iv) Phytoplankton populations in the lake declined initially thereby greatly reducing photosynthesis.

Options

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

Answer

B

Explanation

The use of urea and phosphate fertilisers and pesticide spray to crop fields are passed down to lake during rains through surface runoff. The presence of extra nutrients brings about dense growth of plant and animal life. This phenomenon is called eutrophication. It will also lead to increase in the biomass of algae which causes algal bloom. This results in the drastic decrease in oxygen in the lake, which led to mass scale death of fishes.