Biomagnification of radiocesium in a marine piscivorous fish

Abstract

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.

Bioaccumulation kinetics and exposure pathways of inorganic mercury and methylmercury in a marine fish, the sweetlips Plectorhinchus gibbosus

“…Among the many elements studied so far, the AE of MeHg and radiocesium in fishes are the highest (Ni et al 2000, Zhao et al 2001, Baines et al 2002. For example, the AE of radiocesium range between 78 and 95% in the marine mangrove snappers Lutjanus argentimaculatus (Zhao et al 2001).…”

“…Despite the major differences in the AE and in elimination between MeHg and Hg(II), the biomagnification of MeHg (TTF > 1) is probably caused principally by very efficient assimilation of the prey, even though the elimination rate constant is consistently low. Similarly, the potential biomagnification of radiocesium with a predicted mean TTF of 2.0 (Zhao et al 2001) resulted from a very high AE, although its elimination rate constant was fairly comparable to that of metals such as Cd, Se, and Zn.Although most bioaccumulation studies on the biomagnification of MeHg assume the Hg species is accumulated in fishes mainly in the dietary phase (e.g. by trophic transfer), there is little experimental evidence on the relative importance of aqueous vs dietary uptake under environmentally relevant exposure regimes (Hall et al 1997).…”

Significance of subcellular metal distribution in prey in influencing the trophic transfer of metals in a marine fish

“…The assimilation efficiency (AE) indicates the fraction of ingested metal remaining in the fish body after the undigested materials are evacuated. Many studies have determined the AEs of different metals in a wide range of aquatic organisms, including fish (Reinfelder and Fisher 1994;Zhao et al 2001;Long and Wang 2005). However, mechanisms underlying the dietary assimilation of metals in marine fish are not yet clearly understood.…”

Ecosystem model-based approach for modeling the dynamics of <sup>137</sup>Cs transfer to marine plankton populations: application to the western North Pacific Ocean after the Fukushima nuclear power plant accident

“…Previous works suggested that radiocesium is the only trace element apart from Hg that may be potentially biomagnified along food chains (Harmelin-Vivien et al, 2012;Heldal et al, 2003;Zhao et al, 2001). In our study, the modeled TTFs were generally higher than the unity for all zooplankton groups, showing evidence of biomagnification potential at this trophic level.…”

“…Rates of both radionuclide uptake and loss are known to be affected by species metabolism, and it has been reported that a large part of the radionuclides accumulated by heterotrophic marine biota comes from food (Thomann, 1981;Kasamatsu and Ishikawa, 1997;Zhao et al, 2001;Rowan, 2013). Therefore, the characterization of the radionuclide distribution in these components should be accompanied by ecological information such as species composition in the ecosystem, population densities, rates of primary and secondary production, food ingestion rate, etc.…”