One of the problems with many of the toxic substances is that they build up in the bodies of aquatic organisms to levels which can harm humans that consume them. Uptake of a chemical from the water and from eating other contaminated organisms is called bioaccumulation.

This animation illustrates the process. A toxic substance is emitted from a land-based source and is deposited in a lake. Phytoplankton (free-floating algae) absorb the chemical and are grazed upon by zooplankton (free-floating animals), concentrating the toxicant. The zooplankton are then consumed by small fish which are, in turn preyed upon by larger fish. As the toxicant is transferred up the food chain, its concentration can be magnified by bioaccumulation as much as a million times. Here, bioaccumulation is reflected deepness of the red color and the height of the bar as the chemical moves up the food chain

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