Temporal variation of cesium isotope concentrations and atom ratios in zooplankton in the Pacific off the east coast of Japan

After the Fukushima Daiichi Nuclear Power Plant accident in March 2011, concentrations of cesium isotopes (Cs-133, Cs-134, and Cs-137) were measured in zooplankton collected in the Pacific off the east coast of Japan from May 2012 to February 2015. The time series of the data exhibited sporadic Cs-137 concentration peaks in zooplankton. In addition, the atom ratio of Cs-137/Cs-133 in zooplankton was consistently high compared to that in ambient seawater throughout the sampling period. These phenomena cannot be explained fully by the bioaccumulation of Cs-137 in zooplankton via ambient seawater intake, the inclusion of resuspended sediment in the plankton sample, or the taxonomic composition of the plankton. Autoradiography revealed highly radioactive particles within zooplankton samples, which could be the main factor underlying the sporadic appearance of high Cs-137 concentrations in zooplankton as well as the higher ratio of Cs-137/Cs-133 in zooplankton than in seawater.