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Class 12th Biology 2013 Set3 Delhi Board Paper Solution

Question 25

How have human activities caused desertification? Explain.
OR
How does algal bloom destroy the quality of a fresh water body? Explain.

Answer
The development of the fertile top soil takes centuries. But it can be removed very easily due to human activities like over- cultivation, unrestricted grazing, deforestation and poor irrigation practices, resulting in arid patches of land. When large barren patches extend and meet over time, a desert is created. Now-a-days, desertification is a major problem particulary due to increased urbanisation.
 
OR
 
Ans. Presence of large amounts of nutrients in water causes excessive growth of planktonic (freefloating) algae, called an algal bloom which imparts a distinct colour to the water bodies. Algal blooms cause deterioration of the water quality and fish mortality. Some bloom forming algal are extremely toxic to human beings and animals.