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What is biological magnification ? If the concertration of DDT were is 0.2 ppm in water of a lake ,what would be its likely concentration in fish in the following food chain ? Plankton --- Fish ---- Fish eating birds .

In this question how to calculate concentration of DDT ie.ppm (part per million)

Biological magnification, also known as bio-magnification refers to the increase in the concentration of toxic substances like DDT, chemicals, heavy substances etc. with each successive trophic level of the food chain. The concentration of these toxic substances is highest at the highest trophic level.

Lets take the example of DDT which have been applied on the farm lands. The DDT gets washed oof from the farm land reaches to the water. In water, the small phytoplanktons (Producers) would accumulate certain amounts of DDT in them and then it would be passed on to the next trophic level zooplanktons and then to (fishes) which feed on these zooplanktons. Concentration of DDT would increase with the trophic level. Producers having the minimum concentration while the tertiary consumers would have the highest concentration of DDT accumulation in them because they consume many secondary consumers which in turn consume many primary consumers . This can be illustrated by the following diagram:

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Biological Magnification, also called bioaccumulation -- is the process whereby certain substances such as pesticides or heavy metals move up the food chain.
These substances work their way into rivers or lakes, and are eaten by aquatic organisms such as fish, which in turn are eaten by large birds, animals or humans. The substances, called bioaccumulants, become concentrated in tissues or internal organs as they move up the chain. 
Occurs when compounds are poorly excreted by an organism thereby leading to a buildup of said compound as the organism consumes more and more organisms/substances containing the compound.

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It refers to the process of increase in the concentration of a toxic chemical with increasing trophic level in a food chain.

for example, harmful or poisonous substances such as DDT sprinkled to kill pests on plants enter the food chain. As human beings occupy the top level in any food chain, the maximum concentration of harmful chemicals gets accumulated in the body, the phenonmenon of which is know as biological magnification.

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