Biomagnification is bioaccumulation through ingestion of prey items, causing contaminant concentration to increase with increasing trophic level.
Because of biomagnification low trophic level species (like phytoplancton) have a lower contaminent concentration than high trofic level species (like birds). A contaminant will biomagnify if it is bioaccumulated at every step of the food chain. This means that each step of the food chain has to excrete less of the substance than it takes in. 
- Biology of marine birds. Schreiber, E.A. & Burger, J. (Eds). 2002. Boca Raton, Florida: CRC Press. 722 pp.