Radiocesium is the only trace element apart from Hg that may be potentially biomagnified at the top of the marine planktonic food chain. We quantified the assimilation efficiency from ingested prey, uptake rate from the aqueous phase, and efflux rate of radiocesium in a marine piscivorus fish (the mangrove snapper Lutjanus argentimaculatus). Aqueous 137 Cs exhibited an approximately linear uptake pattern over a 4 d exposure period, and was immediately transported to the muscles. The calculated uptake rate constant (0.00145 l g-1 d-1) was independent of the ambient Cs concentration. Salinity variation appeared to have no influence on the 137 Cs influx within the range of 20 to 30 psu, but the influx rate increased when the salinity was further reduced to 15 psu. The assimilation efficiency in fish ingesting different prey (copepods, Artemia, clam tissues, and herbivorous fish), measured by a pulse-chase feeding technique, ranged between 78 and 95%. The efflux rate constant of 137 Cs in fishes following uptake from the dissolved and dietary phases ranged between 0.020 and 0.023 d-1. The higher efflux rate in marine fishes compared to those in freshwater fishes may have been due to the ionic regulation in marine teleosts (e.g., high excretion rate to counteract the high ambient K + concentration). Using a simple kinetic model, we show that the dietary uptake of 137 Cs plays a dominant role when the concentration factors of 137 Cs in prey range between 50 and 100. At a lower value for the concentration factor (10), 137 Cs bioaccumulation in fish is dominated by uptake from the aqueous phase. The predicted trophic transfer factor (concentration in the predator to concentration in the prey) in the predatory fish ranges between 1 and 4.4 (with a median value of 2), and is consistent with the field measurements of trophic transfer factor of 137 Cs in the piscivorous fishes in both marine and freshwater systems. Thus, the biomagnification of 137 Cs in marine predatory fishes is largely caused by the extremely high 137 Cs assimilation from ingested prey, despite the relatively high efflux rate of 137 Cs compared to those measured in freshwater fishes.

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