Question

A lake near a village suffered heavy mortality of fishes within a few days. Which one of the following reasons are correct?

A. Lots of urea and phosphate fertilizers were used in the crops in the vicinity
B. The area was sprayed with DDT
C. Increase in BOD
D. Phytoplankton populations in the lake declined

• A. A, C
• B. A, B
• C. B, C
• D. C, D

Answer 1

The correct option is A A, C

Lots of urea and phosphate fertilizers used on the crops of the vicinity increase nutrient content of water bodies. The availability of excess nutrients causes algal blooms to emerge in the water body, giving it a greenish tinge. The algae also release toxins in the water making it stinky. This process eventually leads to eutrophication of the water body.
When DDT enters water bodies and consumed by fishes it leads to biomagnification.
Even if phytoplankton populations in the lake decrease it will not directly affect the mortality of fishes in a few days. The fish can still feed on insects or worms.