Question

A lake near a village suffered heavy mortality of fish within a span of a few days. The possible reasons are :-
$a.$ Lot of urea and phosphate fertilizers were used in nearby crops
$b.$ The area was sprayed with DDT by the aircraft
$c.$ The lake water turned green and stinky
$d.$ Phytoplankton population in the lake declined initially thereby greatly reducing photosynthesis

• A. $a,b$
• B. $a,c$
• C. $b,c$
• D. $c,d$

Answer 1

The correct option is B $a,c$

The use of urea and phosphate fertilizers and pesticide spray to crop fields are passed down to lake during rains through surface runoff. The presence of extra nutrients brings about dense growth of plant and animal life. This phenomenon is called eutrophication. It will also lead to increase in the biomass of algae which causes algal bloom. This results in the drastic decrease in oxygen in the lake, which led to mass scale death of fishes.
Hence, this indicates that lot of urea and phosphate fertilizers are used in nearby crops. The lake water could have turned green and stinky due to algal bloom.
So, the correct statements are - a and c.