Question

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

1. lots of urea and phosphate fertilisers were used in the crops in the vicinity 
2. the area was sprayed with DDT by an aircraft 
3. the lake water turned green and stinky 
4. phytoplankton populations in the lake declined initially thereby greatly reducing photosynthesis Which two of the above were main reasons of fish mortality in the lake?

Options

• a, c

• a, b

• b, c

• c, d

Answer 1

a, c

Explanation:

Fertiliser runoff adds nutrients to the lake, causing eutrophication and an algal bloom; the green, foul-smelling water is evidence of this process. Oxygen depletion during decomposition kills fish, whereas DDT spraying and an initial decline in phytoplankton are not the main reasons given.