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Match column I with column II and select the correct option regarding biomagnification of DDT in an aquatic food chain.

Column I (Organism from aquatic food chain) Column II (Level of DDT in the body)
I. Small fish P. 25 ppm
II. Fish-eating birds Q. 2 ppm
III. Zooplankton R. 0.5 ppm
IV. Large fish S. 0.04 ppm

I - S, II - Q, III - R, IV - P
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I - R, II - S, III - Q, IV - P
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I - R, II - P, III - S, IV - Q
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I - S, II - R, III - P, IV - Q
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The correct option is C I - R, II - P, III - S, IV - Q
Biomagnification is the phenomenon through which certain pollutants increase in concentration at successive trophic levels along the food chain.

Many of the insecticides are non-biodegradable and their residues have a long life.

One common example is DDT (dichlorodiphenyltrichloroethane). Scientist Paul Muller discovered the use of DDT as an insecticide (a toxic substance that is used to kill insects).

It was sprayed on water bodies to kill insects in order to combat malaria (mosquito-borne) and the other insect-borne human diseases among both military and civilian populations during World War II.

However, DDT present in water often undergoes biomagnification in the aquatic food chain. Once it is absorbed by the organisms, it cannot be metabolised, broken down or excreted out by the body of the organisms. This normally accumulates in the fat-containing tissues of the organisms and is passed on to the next higher trophic level and its concentration increases at successive trophic levels.

Consider an aquatic food chain: zooplankton → small fish → large fish →fish-eating birds.

It has been observed that the concentration of the DDT progressively increases along the food chain from zooplankton to fish-eating birds through biomagnification.

If the concentration of DDT is 0.003 ppb in water (parts per billion: one microgram of DDT per litre of water), then the concentration of DDT in zooplankton is 0.04 ppm (parts per million: one milligram of DDT per litre of water). The concentration of DDT in small fishes that feed on the zooplankton is 0.5 ppm. The concentration of DDT progressively increases in the body of large carnivorous fishes which feed on small fishes and reaches to 2 ppm.

The DDT concentration ultimately reaches to 25 ppm in fish-eating birds through biomagnification.

DDT will disturb the calcium metabolism in birds, which causes thinning of eggshells and their premature breaking, eventually causing decline in the bird population.

Figure: Biomagnification of DDT in an aquatic food chain

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