A Bioaccumulation is refers to the process by which contaminants such as PCBs accumulate or become magnified as they move up the food chain. PCBs concentrate in tissue and internal organs, and as big fish eat little fish, they accumulate all the PCBs that have been eaten by everyone below them in the food chain. Another term for this is Biological Magnification.

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Other /More definition:
A Bioaccumulation is the retention and buildup of chemicals or hazardous substances in the bodies of organisms due to repeated exposure or consuming contaminated organisms lower on the food chain