Explain biomagnification or biological magnification. 

Explanation

Toxic wastes present in industrial wastes and water from farmhouse containing pesticides and weedicides enters the food chain of aquatic organisms.
The increase in concentration of toxicant at each successive trophic levels is called biological magnification.
The most common toxicant that get accumulated at successive trophic levels includes DDT and Mercury. 

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