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 an aircraft.
(c) Lake water turned green and stinky.
(d) Phytoplankton population declined reducing photosynthesis.
A. a, b
B. a, c
C. b, c
D. c, d

Answer 1

Hint:Eutrophication is a condition which is responsible for the death of the fishes in lakes. In this condition, the amount of nutrition and minerals present in water is excessive which leads to excessive growth of algae. It leads to a decrease in the amount of oxygen present in the water causing fishes to die.

Complete answer:
Lake near the village suffered heavy mortality of fishes because of the condition known as eutrophication.
Eutrophication can be caused due to natural and man-made reasons.
- Man-made or human interference can lead to eutrophication of a lake and is called cultural eutrophication.
- In this type of eutrophication the land activities near the lake like farming, industrial waste discharge, etc., can lead to eutrophication.
- Farmers use a lot of fertilizers in their crops which contain phosphate and urea. These fertilizers usually get mixed up with water during irrigation and travel to the lake. Thus depositing phosphate and urea in the lake causing eutrophication.
- Due to the presence of phosphate in lakes, there is a growth of excess of algae and phytoplankton.
- Algae when in excess amounts start to consume oxygen present in water causing its depletion for fishes. The absence of oxygen leads to the death of the fishes.
- Phytoplankton on the other hand decreases the clarity of the water. It reduces the amount of sunlight that reaches the bottom of the lake. It is also responsible for the depletion of oxygen levels in the lake. Thus, causing a negative effect on the organism living in the lake.

So, the answer is option (B) - ‘a,c’

Note:
The accumulation of DDT can lead to eutrophication. It causes an increase in BOD which leads to a decrease in the amount of oxygen level present in the water. Accumulation of DDT in fishes that get consumed by humans can cause Minamata disease.
Human interference can cause the fastening of eutrophication. Therefore, to protect lakes it is advised to use natural or phosphate-nitrogen free fertilizers. The industrial waste should be treated before discharging into the lakes and rivers. Washing of clothes with phosphate-containing detergents should be avoided.