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, Cs, and Cs) 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 concentration peaks in zooplankton. In addition, the atom ratio of Cs/Cs 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 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 concentrations in zooplankton as well as the higher ratio of Cs/Cs in zooplankton than in seawater.