A lake near a village suffered heavy mortality of fishes within a few days. Consider the following reasons for this: (i) Lots of urea and phosphate fertilizer were used in the crops in the vicinity (ii) The area was sprayed with DDT by an aircraft (iii) The lake water turned green and stinky (iv) 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?
- (i), (ii)
- (iii), (iv)
- (i), (iii)
- (ii), (iii)
Answer
(1) (i), (ii)
Solution
(i) Run-off of urea and phosphate fertilizers adds nutrients to the lake eutrophication and algal bloom; decomposition of the algae depletes dissolved oxygen fishes die.
(ii) DDT sprayed over the area is washed into the lake; DDT is highly toxic to fishes and, as per NCERT, it is biomagnified in aquatic food chains fishes die.
(iii) Green, stinky water is only a result (symptom) of eutrophication, not a cause by itself.
(iv) is incorrect: nutrient enrichment increases phytoplankton initially; it does not decline.
The main causes of fish mortality are (i) and (ii).
Answer: (1)