Contrasting radiocesium transfer in the river and lake food webs: Importance of trophic level and food source

Radiocesium (137Cs) contamination of the freshwater ecosystems adjacent to the Fukushima Daiichi Nuclear Power Plant (FDNPP) in Japan has persisted long after the accident that occurred at the facility in March 2011. It is necessary to elucidate the dynamics of 137Cs in various aquatic ecosystems to predict 137Cs concentrations in fish and manage freshwater fisheries in the vicinity of FDNPP. To these ends, we applied stable isotope analysis to evaluate changes in 137Cs levels through trophic positions and the relative importance of the 137Cs sources at the trophic bases of two rivers and two lakes in Fukushima. The δ15N analyses disclosed that 137Cs decreases from primary producers to fish consumers in the river food web and 137Cs increases among fish consumers with increasing trophic position in the lake food web. The δ13C analysis revealed that autochthonous 137Cs contributed to fish contamination. The periphyton-dependent and zooplankton-dependent fish had comparatively higher 137Cs concentrations in the rivers and lakes, respectively. Cesium-137 supply from the pelagic food web was observed to contribute to greater 137Cs levels in the fish consumers inhabiting the lakes. The results of this study show that stable isotope analysis may help clarify 137Cs dynamics in freshwater food webs and identify the important 137Cs sources in the food web. Identifying important 137Cs sources and trophic transfers depending on the ecosystem help guide regulatory and management frameworks to establish profitability of the food fish stocks there and maintain food security.