Modeling radiocesium bioaccumulation in a marine food chain

We measured the transfer of radiocesium in a marine food chain from phytopl ankton to bivalves and finally to a predatory gastropod (Babylonia formosae habei). The assimilation efficiency (AE) of radiocesium in both green muss els (Perna viridis) and the gastropods feeding on different diets was measu red by a pulse-chase feeding radiotracer technique. The AEs of Cs-137 in th e green mussels ranged between 0.4 and 10%, and were the lowest for mussels feeding on natural sediment. The bioconcentration factor of Cs-137 ranged between 10 and 120 1 kg(-1) for 2 different phytoplankton species (diatom T halassiosira pseudonana and green alga Chlorella autotrophica). The AEs of Cs-137 in the predatory gastropods were 44 to 58 % of those feeding on muss els and clams (Ruditapes philippinarum). The efflux rate constant of Cs-137 in the gastropods was 0.074 d(-1). Using a simple kinetic model, we showed that the majority of Cs-137 in the mussels was due to uptake from the diss olved phase primarily because Cs-137 is not particle-reactive. Uptake due t o ingestion of particulate materials contributed Little to the overall Cs-1 37 accumulation in the mussels, except when the resuspended sediment consti tuted a major food source for mussels. In contrast, dietary ingestion (trop hic transfer) can be an important source for radiocesium accumulation in th e predatory gastropod because of its efficient assimilation. Our modeling r esults indicated that the trophic transfer factor was <1 in both bivalves a nd gastropods. Consequently, Cs-137 was not biomagnified during its transfe r to filter-feeding bivalves and predatory gastropods. This was primarily d ue to the high turnover rate of radiocesium in both bivalves and gastropods , even though the AE of radiocesium in the predators was high. However, the trophic transfer factor tended to increase with increasing trophic level, and a factor close to 1 may be reached when ingestion of the animals is hig h.