Question

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 fertilisers were used in the crops in the nearby fields.
(ii) The croplands of the village were sprayed with DDT.
(iii) The lake water turned green and stinky.
(iv) Phytoplankton populations in the lake declined initially thereby greatly reducing photosynthesis.
A. (i) and (iii)
B. (i), (ii) and (iii)
C. (ii), (iii) and (iv)
D. (iii) and (iv)

Answer 1

Hint: Increase in algal blooms in the crops can lead to blockage of the whole surface by the alga and reduction of oxygen available to water animals and their death due to lack of oxygen.

Complete answer:
If in a nearby village a heavy mortality rate among the fishes is experienced within a few days then that could be due to various reasons. It can be due to release of urea and phosphate based fertilizers into the river bodies when they are used in the nearby crops. Chemicals like urea and phosphate pollute the water bodies nearby causing damage to water quality and increase in algal blooms. Furthermore, if the crops in the fields of the village are sprayed with DDT which is a harmful chemical if released in water bodies, the content of chemicals increases in the water bodies and they become more polluted. With the further increase in algal blooms in the water bodies, there is reduction in oxygen supply to the animals and fishes that live in these water bodies leading to their deaths. Because of this the water bodies turn green and stinky.

Therefore the statements (i), (ii) and (iii) are correct.

Option B: This is the correct option.

Option A, C & D: They are the incorrect options.

Hence, the correct answer is option (B)

Note: For aquatic animals to survive in water, it is essential that the water supply is adequate and continuous. With the increase in algal bloom the whole surface of water bodies is occupied leading to death of fishes.