Trophic transfer of seven trace metals in a four-step marine food chain

There is increasing recognition of the importance of dietary pathways in determining metal body burdens in marine organisms. With a simple kinetic model that requires information about the ingestion rate of an animal and the assimilation efficiency (AE) and efflux rate constant (ke) of a metal following dietary exposure, it is possible to quantitatively predict the trophic transfer and biomagnification potential of a metal between trophic levels. In this study, we used radiotracers to examine the trophic transfer of 7 metals (Am, Cd, Co, Cs, Mn, Se, and Zn) in a 4-step marine food chain from phytoplankton (Isochrysis galbana) to crustacean zooplankton (Artemia salina) to juvenile sea bream (Sparus auratus) and finally to piscivorous sea bass (Dicentrarchus labrax). AEs generally were highest for Cs (63–84%) and Se (60–77%), and lowest for Am (<10%) for all animals; differences in AEs for the other metals were comparatively small for all animals. There was no consistent pattern among the metal kes; however, kes for any given metal tended to decrease with increasing trophic level. At each trophic step, we calculated the trophic transfer factor (TTF), defined mathematically as the ratio of metal concentration in predatory animals to metal concentration in prey organisms at steady state, and found that this ratio consistently approached or exceeded 1 for only Cs, suggesting that Cs biomagnifies at every trophic step from phytoplankton to fish. TTF values were always <1 for Am, Cd, Mn, and Co, suggesting that these metals are not expected to biomagnify in marine food chains. Se and Zn did not consistently display TTF values >1 at every trophic level, but values were close enough to unity to suggest the possibility of biomagnification under certain environmental regimes.