Question

Fishes died within a few days in a lake near a village. Following are the possible reasons for this:

(a) Lot of urea and phosphate fertilisers used in crops in the vicinity

(b) Spraying agricultural fields with degradable pesticides

(c) Lake water turned green and stinky

(d) Phytoplankton population declined in the lake

What are the two main causes of fish mortality in the lake?

• A. (a), (c)
• B. (a), (b)
• C. (b), (c)
• D. (c), (d)

Answer 1

The correct option is A (a), (c)

$\bullet$ When excess nitrogen or phosphorus fertilizers are applied on crops, it seeps into the water bodies nearby and it becomes a source of nutrient for the algae and aquatic plants, boosting its growth.
$\bullet$ Algae then spread and cover the water surface, depriving underwater plants of sunlight.
$\bullet$ The algae eventually dies and bacteria start the process of decomposition. This process depletes the dissolved oxygen causing the death of fishes and other aquatic organisms.
$\bullet$ Whereas, degradable pesticides are ecofriendly.