Assertion : The phenomenon of biomagnification is well known for mercury and DDT .

Reason : High concentrations of DDT disturb calcium metabolism in birds .

The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

A few toxic substances, often present in industrial waste waters, can undergo biological magnification (Biomagnification) in the aquatic food chain.

Biomagnification refers to increase in concentration of the toxicant at successive trophic levels. This happens because a toxic substance accumulated by an organism cannot be metabolised or excreted, and is thus passed on to the next higher trophic level.

This phenomenon is well- known for mercury and DDT.

High concentrations of DDT disturb calcium metabolism in birds, which causes thinning of eggshell and their premature breaking, eventually causing decline in bird populations.