Evaluating the trophic transfer potential of metals in marine and estuarine food chains

The sources, fate, and bioaccumulation of metals have received increasing attention recently as a consequence of human and wildlife concerns resulting from the consumption of metal-contaminated fish.;I examined the trophic transfer of Am, Co, Cd, Cs, Mn, Se, and Zn from the phytoplankton Isochrysis galbana to the crustacean zooplankton Artemia salina to juvenile sea bream Sparus auratus to 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 efflux rates of metals from the fish (kes), however kes for any given metal tended to decrease with increasing trophic level. TTFs consistently approached or exceeded 1 only for Cs, suggesting that Cs biomagnifies at every trophic step from phytoplankton to fish. TTF values were always less than 1 for Am, Cd, Mn, and Co, suggesting that these metals would not be 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.;To assess the extent to which the observed patterns in metal accumulation were species-specific, I compared the assimilation and retention of Am, Co, Cs, Cd, Mn and Zn in teleosts (Psetta maxima and Sparus auratus) and an elasmobranch fish (Scyliorhinus canicula ). Mean AEs of ingested metals in the predator fish ranged from 6 to 15% for Am to 63 to 74% for Cs; significant differences in AEs were noted between species for Co and Mn only. The kes of assimilated metal did not differ significantly among predators for any metal. Observed differences in tissue metal concentrations between teleosts and elasmobranches may therefore be due to the different growth and ingestion rates as well as the reproductive strategies and dietary preferences of these two groups of fish.;I demonstrated through the previously described laboratory experiments and model results that dietary exposure accounts for > 90% of the total body burden of most metals considered in marine fish. Dietary pathway should therefore have an effect on metal accumulation dynamics in fish and should affect the potential for a metal to biomagnify in a given organism. I explored the relationship between body size, feeding ecology and metal concentration in the hake Merluccius merluccius collected from the Gulf of Lions in the Mediterranean Sea and found a negative correlation between size and metal levels for essential metals. This correlation is likely due in part to ontogenetic shifts in diet preference between juvenile and adult hake, and in part to elevated ingestion rates and metal uptake rates observed in smaller-sized fish.;I examined the trophic transfer of Cd, Po, and methyl-Hg in an estuarine food chain from phytoplankton (the diatom Cyclotella meneghiniana and the chlorophyte Chlamydomonas reinhardtii) to the cladoceran zooplankton Daphnia pulex to the killifish Fundulus heteroclitus to juvenile striped bass Morone saxatilis . The resulting assimilation efficiencies (AEs) of ingested metals in all animals were highest for MeHg, with values > 76%. I showed that the trophic transfer factor was greater than unity for all three metals in Daphnia feeding on phytoplankton, suggesting that these metals may be biomagnified at this trophic step. In killifish feeding on Daphnia , the trophic transfer factor (TTF) was consistently >1 for MeHg, consistently < 1 for Cd, and ranged between 0.1 -- 1.4 for Po depending on AEs and ingestion rates of the killifish, suggesting that both MeHg and Po have the potential to biomagnify during trophic transfer from Daphnia to killifish.;My field data was largely consistent with the previously described laboratory studies, in that there was no clear evidence of biomagnification of any of the metals studied but As and Se showed a tendency to build-up in hake with increasing food chain length. As was the only element for which concentrations were higher in adult hake than in juveniles. The trophic level of hake was determined using stable isotopes of nitrogen, and we found that in stations where adult hake occupied a higher trophic level, tissue metal concentrations were higher in Se and As.;While many metals may not biomagnify in fish under most scenarios in noncontaminated marine and estuarine systems, the accumulation of metals to harmful levels may still threaten wildlife and human consumers of fish even in the absence of biomagnification. Understanding of the processes governing trophic transfer is crucial, and these findings underscore the importance of including dietary exposure in establishing appropriate water quality criteria and in risk assessments for metals in aquatic ecosystems. (Abstract shortened by UMI.).