Modeling radiocesium bioaccumulation in a marine food chain

We measured the transfer of radiocesium in a marine food chain from phytoplankton to bivalves and finally to a predatory gastropod (Babylonia formosae habei). The assimilation efficiency (AE) of radiocesium in both green mussels (Perna viridis) and the gastropods feeding on different diets was measured by a pulse-chase feeding radiotracer technique. The AEs of 137Cs in the green mussels ranged between 0.4 and 10%, and were the lowest for mussels feeding on natural sediment. The bioconcentration factor of 137Cs ranged between 10 and 120 l kg–1 for 2 different phytoplankton species (diatom Thalassiosira pseudonana and green alga Chlorella autotrophica). The AEs of 137Cs in the predatory gastropods were 44 to 58% of those feeding on mussels and clams (Ruditapes philippinarum). The efflux rate constant of 137Cs in the gastropods was 0.074 d–1. Using a simple kinetic model, we showed that the majority of 137Cs in the mussels was due to uptake from the dissolved phase primarily because 137Cs is not particle-reactive. Uptake due to ingestion of particulate materials contributed little to the overall 137Cs accumulation in the mussels, except when the resuspended sediment constituted a major food source for mussels. In contrast, dietary ingestion (trophic transfer) can be an important source for radiocesium accumulation in the predatory gastropod because of its efficient assimilation. Our modeling results indicated that the trophic transfer factor was <1 in both bivalves and gastropods. Consequently, 137Cs was not biomagnified during its transfer to filter-feeding bivalves and predatory gastropods. This was primarily due 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 high.