Fukushima-derived radionuclides 134Cs and 137Cs in zooplankton and seawater samples collected off the Joban-Sanriku coast, in Sendai Bay, and in the Oyashio region

The Fukushima Dai-ichi nuclear power plant (FNPP) accident that followed the Great East Japan Earthquake in 2011 resulted in the release of enormous quantities of anthropogenic radionuclides, especially radioactive cesium (Cs-134 and Cs-137) into the ocean off the east coast of Japan. FNPP-derived radioactive Cs may have consequently accumulated within marine food webs via seawater intake and predator-prey interactions. We provide evidence of the temporal variability in Cs-134 and Cs-137 concentrations in seawater and zooplankton samples collected in coastal waters off Joban-Sanriku, in Sendai Bay, and in the Oyashio region between June 2011 and December 2013. In Sendai Bay, seawater Cs-134 and Cs-137 concentrations exceeded 1 Bq/kg in June 2011 and rapidly decreased during the study period. Cs-134 and Cs-137 concentrations in zooplankton were also high in June 2011, up to 23 Bq/kg-wet and also decreased during the study period, although at a slower rate than seawater Cs-134 and Cs-137 concentrations. Regarding Cs-137 concentrations, the difference in the rate of decrease between seawater and zooplankton resulted in a high apparent concentration ratio (aCR) for zooplankton. The observed relation between Cs-137 in seawater and the aCR of zooplankton were good indicators of the progress of Cs-137 contamination in zooplankton from the beginning of the FNPP accident to the restoration phase.