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. II, III
B. III, IV
C. I, III
D. I, II

Answer

D

Explanation

A lake near a village suffered heavy mortality of fishes within a few days, because lots of urea and phosphate fertiliser were used in the crops in the vicinity and the area was sprayed with DDT by an aircraft. Inorganic phosphorus and nitrogen are responsible for the growth of algae.
In polluted water, these increase due to which algae increase greatly at the surface of water forming water bloom. Due to the death of these algae their organic matter gets decomposed due to which oxygen gets depleted and aquatic animals die.