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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 fertilizer 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 the main causes of fish mortality in the lake?

1. (2) and (3)
2. (3) and (4)
3. (1) and (3)
4. (1) and (2)

Correct answer: (1) and (3)

Solution

Excessive use of urea and phosphate fertilizers (1) leads to eutrophication, causing algal blooms that deplete oxygen in water, making it green and stinky (3). This oxygen depletion is the primary cause of fish mortality.

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