Q. A lake near a village suffered heavy mortality of fishes within a few days. Consider the following reasons for this (a) Lots of urea and phosphate fertilizer were used in the crops in the vicinity (b) The area was sprayed with DDT by an aircraft (c) The lake water turned green and stinky (d) 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?
A) a, b
B) b, c
C) c, d
D) a, c
Explanation
[Solution from source] Fertiliser runoff rich in nitrates and phosphates triggers algal blooms (the water turns green), and when the algae die their decomposition uses up dissolved oxygen and gives off foul smells, killing fish within days. DDT biomagnifies slowly over time and would not explain sudden mass death alongside these signs, and phytoplankton increase rather than decline. Answer (4).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.