Biomagnification is the accumulation of the increasing amount ? of non-degradable pollutants through the food chain i.e., the concentration of toxicants increases at successive trophic levels.It is caused because a toxic substance accumulated in an organism cannot be metabolized or excreted out.It can only be passed on to the next trophic level, in which the concentration of the toxicant increases.It is observed in aquatic food chains.An aquatic food chain can be represented as:Phytoplankton ? Zooplankton ? Small Fish ? Large Fish ? Fish-eating birdsPhytoplanktons are the producers in the food chain, which are eaten by the zooplanktons and successively by fishes and then fish-eating birds.Biomagnification is a well-known phenomenon for toxicants like mercury and DDT, which get released into water bodies.Explanation:If we take the example of DDT, it starts with 0.003 ppb (parts per billion) in water.Its concentration is found to increase to 0.04 ppm (parts per million) in zooplanktons.It keeps increasing in successive trophic levels and we can find up to 25 ppm of DDT in the fish-eating birds.Such high concentrations of DDT disturbs the calcium metabolism in birds.It may cause thinning of eggshell and premature breaking of eggs, which eventually leads to decline in bird populations.