What is biomagnification?
Biomagnification is the increase in concentration of one or more toxic substance(s) on progressive levels of the food chain.
The way this occurs is that lower trophic levels or producers (i.e. plants and algae) pick up minimal amounts or residues of toxins from the environment. Organisms who feed on these plants may also ingest the toxins with it (assuming that it is very stable) and by virtue of the consumer feeding on a large amount of these producers, the amount of toxic substance it eats and retains increases.
This trend continues as we go to higher trophic levels whereas apex consumers usually end up eating organisms that have built up hazardous concentrations of these toxins which then would have adverse effects to the top consumer and may inadvertently lead to upsetting the ecological balance.