Question

The excessive use of chemical fertilisers in the fields cause
a. Eutrophication
b. Decline in soil fertility
c. Biomagnification
d. None of the above

Answer 1

The streams draining the fertilisers into the lake introduce nutrients such as nitrogen and phosphorus, which encourage the growth of algae. The algal bloom occurs due to increases in the lake's fertility. The aquatic equilibrium is often disturbed by deterioration of water quality and death of aquatic plants and animals like fishes which is caused by algal bloom.

Complete answer:
- The natural aging of a water body due to nutrient enrichment of its water is called Eutrophication.
- As the time passes, streams draining into the lake introduce nutrients such as nitrogen and phosphorus, which encourage the growth of aquatic organisms.
- Plant and animal life burgeons, and organic remains begin to be deposited on the lake bottom as the lake’s fertility increases.
- As silt and organic debris pile up, the lake becomes shallower and warmer.
- The warm-water organisms suppress the growth of those who prefer a cold environment. Thus marsh plants take root in the shallows and hence they begin to fill the original lake basin.
- Finally, the lake gives way to large masses of floating plants (bog), which finally converts water into land.
- A large number of pollutants from man’s activities like effluents from the industries and homes readily accelerate the aging process of water bodies which is known as Cultural or Accelerated Eutrophication.

The excessive use of chemical fertilisers in the fields cause Eutrophication.
Hence, the correct answer is option (A).

Additional information:
The Government has realised the importance of maintaining cleanliness of water bodies and passed the Water Prevention and Control Act, 1974, to safe water resources.

Note: Domestic sewage reduces dissolved oxygen but increases biochemical oxygen demand of receiving water. The rich source of pollutants like nitrogen and phosphorus, which cause eutrophication and nuisance creating algal blooms are domestic sewage. On the other hand industrial waste waters are often rich in toxic chemicals, especially heavy metals and organic compounds.