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(a) What depletes ozone in the strastophere? How does this affect human life?

(b) Explain biomagnification od DDT in an aquatic food chain. How does it affect the

bird population?


(a) The thin layer of of ozone around the atmosphere that prevents entry of harmful

ultra-violet rays (UV-rays) is called ozone shield. Methane and chloroflurocarbons are

the two ozone depleting substances.

Chloroflurocarbons release active chlorine when UV rays act on them in the stratosphere.

Cl-atoms degrade ozone releasing molecular oxygen. Depletion of ozone allows the entry of 

UV radiation to the earth.

UV-B rays damage DNA and may also cause mutation. UV- radiation causes aging of skin,

damages skin cells and causes various types of cancers. It also leads to inflammation of 

cornea called snow-blindness, cataract etc.

(b) Biological magnification or biomagnification is the increase in concentration of toxic 

materials like heavy metals and pesticides at each trophic level of a food chain due to 

their accumulation. A toxic substance accumulated by an organism can not be metabolised

or excreted and is passed on to the next higher trophic level.

Water(DDT 0.03 ppm)Zooplankton(DDT 0.04 ppm)Small fish(DDT 0.5 ppm)Large fish(DDT 2 ppm)Bird(DDT 5 ppm)

The population of fish-eating birds living on the bank of a contaminated lake differs

from those living on the bank of another lake free from such insecticides. DDT being

highly poisonous disturbs calcium metabolism in birds which causes the thinning of 

their egg-shells and premature breaking thereby causing a decline in their population.