Biological Magnification – The increase in the concentration of harmful chemical substances such as pesticides, in the body of living organisms at each trophic level of a food chain is called biological magnification. This increase may occur as a result of

1. Persistence – The substance cannot be broken by environmental processes.
2. Food chain energetics – The substance concentration increases progressively as it moves up a food chain.

Here is an example of biological magnification. Suppose the water in a lake of found to contain 0.02 ppm of pesticides. The phytoplankton (the diverse collection of organisms that live in large bodies of water and are unable to swim against a current), which flourished in this water retained 5ppm of pesticides in their bodies. The fish, which ate these phytoplanktons had 240 ppm of pesticides and thereafter the birds, which consumed these fishes had as much as 1600 ppm pf pesticides in their bodies. Thus, the concentration of pesticides in the body of living organisms gradually increases, that is, the pesticides get magnified.

In short, in biological magnification, pesticides or heavy metals move up the food chain, work their way into rivers or lakes are eaten by aquatic organisms such as fish, which is turn by large animals, birds, or humans.