Bioaccumulation is the accumulation of contaminants by species in concentrations that are orders of magnitude higher than in the surrounding environment.

Bioaccumulation is the sum of two processes: bioconcentration and biomagnification. Bioconcentration is the direct uptake of a substance by a living organism from the medium (e.g., water) via skin, gills, or lungs, whereas biomagnification results from dietary uptake. Many synthetic contaminants are more soluble in fat than in water. Polychlorinated biphenyls (PCBs), for example, which can be present in lake or river water, tend to either adsorb to particles or to diffuse into cells of organisms. Thus, PCBs bioconcentrate in low trophic levels, for example, in phytoplankton by a factor of around 250. Fish that actively filter large amounts of water through their gills are subject to a much higher bioconcentration. Additionally, biomagnification takes place in predatory organisms. The PCB burden of the prey is transferred to...