Bioaccumulation is the build up of toxic chemical pollutants in the tissues of organisms. Toxic contaminants often cannot be easily metabolized or excreted, and therefore are bioaccumulated at high levels within an organism. Bioaccumulation of toxic chemicals can result in biomagnification, an increase in the contaminant at higher levels of the food web.

Chemical pollutants that are bioaccumulated come from many sources. Pesticides are an example of a contaminant that bioaccumulates in organisms. Rain can wash freshly sprayed pesticides into creeks, where it will eventually make its way to rivers, estuaries and the ocean. Other major sources of toxic contaminants include atmospheric deposition with rainfall (initially from industrial smokestacks and automobile emissions), and both legal and illegal discharges to water. For example, industrial discharges can be major sources of toxic contaminants. Sludge from sewage treatment plants has been a source in the past.

Once a toxic pollutant is...