From MarineSpecies Introduced Traits Wiki
Biomagnification is bioaccumulation through ingestion of prey items. It causes contaminant concentration to increase with increasing trophic level. Because of biomagnification low trofic level species (like phytoplancton) have a lower contaminent concentration than high trophic level species (like birds). .
A contaminant will biomagnify if it is bioaccumulated by every step of the foodchain. 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.