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

Radiocesium (Cs) 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 Cs in various aquatic ecosystems to predict Cs concentrations in fish and manage freshwater fisheries in the vicinity of FDNPP. To these ends, we applied stable isotope analysis to evaluate changes in Cs levels through trophic positions and the relative importance of the Cs sources at the trophic bases of two rivers and two lakes in Fukushima. The δN analyses disclosed that Cs decreases from primary producers to fish consumers in the river food web and Cs increases among fish consumers with increasing trophic position in the lake food web. The δC analysis revealed that autochthonous Cs contributed to fish contamination. The periphyton-dependent and zooplankton-dependent fish had comparatively higher Cs concentrations in the rivers and lakes, respectively. Cesium-137 supply from the pelagic food web was observed to contribute to greater Cs levels in the fish consumers inhabiting the lakes. The results of this study show that stable isotope analysis may help clarify Cs dynamics in freshwater food webs and identify the important Cs sources in the food web. Identifying important Cs 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.