Temporal Changes in 137Cs Concentration in Zooplankton and Seawater off the Joban–Sanriku Coast, and in Sendai Bay, After the Fukushima Dai-ichi Nuclear Accident

Abstract

The Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident following the Great East Japan Earthquake in 2011 resulted in the release of enormous quantities of anthropogenic radionuclides into the ocean off the east Japanese coast, especially radioactive cesium (134Cs and 137Cs). FNPP-derived radioactive Cs might have consequently accumulated within marine food webs via seawater intake and predator–prey interactions. This study provides evidence of temporal variability in 137Cs concentrations in seawater and zooplankton samples collected off the Joban–Sanriku coast and in Sendai Bay between June 2011 and December 2013. In Sendai Bay, seawater 137Cs concentration was more than 1 Bq/kg in June 2011 and rapidly decreased over the study period. 137Cs concentration in zooplankton was also measured to be as high as high 23 Bq/kg-wet in June 2011, and this concentration decreased at a slower rate than seawater concentrations. The difference in the rate of decrease of 137Cs concentration between seawater and zooplankton resulted in an elevated apparent concentration ratio (aCR) for zooplankton. The observed relationship between 137Cs in seawater and the aCR of zooplankton reflected the progression of 137Cs contamination in zooplankton from the beginning of the FNPP accident to the restoration phase.