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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) 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?
A
(i), (iii)
B
(i), (ii)
C
(ii), (iii)
D
(iii), (iv)
Detailed Solution
(i) Urea and phosphate fertilisers used in the nearby crop fields are washed into the lake with run-off water. They enrich the water with nitrates and phosphates.
This nutrient enrichment causes an excessive growth of planktonic algae, called an algal bloom, which is the process of accelerated (cultural) eutrophication.
(iii) The algal bloom turns the water green and gives it a foul smell. When the algae die, decomposer microbes break them down and consume the dissolved oxygen of the water; the biochemical oxygen demand rises sharply.
The fall in dissolved oxygen, together with toxins released by some bloom-forming algae, causes heavy mortality of fishes within a few days.
(ii) DDT undergoes biomagnification over a long period and mainly harms the top consumers such as fish-eating birds; it does not cause such sudden mass death of fishes.
(iv) is opposite to what happens: the phytoplankton population increases greatly instead of declining.
So the two main causes are (i) and (iii).
This nutrient enrichment causes an excessive growth of planktonic algae, called an algal bloom, which is the process of accelerated (cultural) eutrophication.
(iii) The algal bloom turns the water green and gives it a foul smell. When the algae die, decomposer microbes break them down and consume the dissolved oxygen of the water; the biochemical oxygen demand rises sharply.
The fall in dissolved oxygen, together with toxins released by some bloom-forming algae, causes heavy mortality of fishes within a few days.
(ii) DDT undergoes biomagnification over a long period and mainly harms the top consumers such as fish-eating birds; it does not cause such sudden mass death of fishes.
(iv) is opposite to what happens: the phytoplankton population increases greatly instead of declining.
So the two main causes are (i) and (iii).