Class 12th Biology 2013 Set3 Delhi Board Paper Solution

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.

 

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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.