Journal Article Temporal changes of Fukushima-derived radiocaesium in pelagic oceanic zooplankton in the western North Pacific.

Kitamura, Minoru  ,  C. Honda, Makio  ,  Hamajima, Yasunori  ,  Kumamoto, Yuichiro  ,  Aoyama, Michio  ,  Kawakami, Hajime  ,  Aono, Tatsuo  ,  Fukuda, Miho  ,  Mino, Yoshihisa

Temporal changes of contamination with the Fukushima-derived radiocesium in oceanic zooplankton were investigated from one month to three years after the Fukushima Dai-ichi nuclear power plant accident. Sampling sites were located in subtropical and subarctic areas (900 and 1900 km away from the nuclear power plant, respectively) of the western North Pacific. 134Cs and 137Cs were detected from all of the time-series samples with the maximum activity concentrations, 70.6 and 71.5 Bq kg-dry–1 for 134Cs and 137Cs, respectively, recorded in the subtropical site on May 2011. In the subtropical zooplankton, activities of 137Cs rapidly decreased by 87–92% from May to July 2011, thereafter activities were maintained one order of magnitude higher than the pre-accident level (180 mBq/kg-dry) through the study period. In the subarctic zooplankton, 137Cs activities with two order of magnitude higher than the pre-accident level were observed until July 2011, and the activities were gently decreased to the pre-accident level on July 2014. The different decreasing rates of radiocesium in zooplankton until first summer after the accident are probably due to different life spans of zooplankton (short in subtropical while long in subarctic). Radiocesium activities in seawater derived by atmospheric deposition rapidly decrease by dispersion in the oceanic area. So, most of the zooplankton that took radiocesium just after the accident might be replaced by newly hatched zooplankton until first summer in the subtropical site, while subarctic zooplankton highly contaminated with radiocesium probably still alive until first summer. Unexpectedly, activities of radiocesium in surface zooplankton (collected from surface mixed layer) were lower than those in subsurface zooplankton (between thermocline to 200 m) in the subarctic site although opposite relationship were observed in activities of radiocesium in seawater. Trophic levels of zooplankton, which were estimated from δ15N of primary producers and zooplankton, suggests that carnivore species are more dominant in the subsurface community. Thus, the difference of activity concentrations of radiocesium between surface and subsurface zooplankton are thought to be influenced by bioaccumulation. Since the first winter after the accident, 137Cs activities in zooplankton were maintained to be high in the subtropical site while those decreased to the pre-accident level in the subarctic site. The former was probably influenced by secondary supply of radiocesium into subtropical site due to advection of the subtropical mode water highly contaminated with the Fukushima-derived radiocesium. We can conclude that radiocesium activity concentrations in zooplankton and their temporal changes are influenced by not only supply of radiocesium into environment but also characters of zooplankton community. Comparing with the previous studies, activity concentrations of radiocesium in our oceanic zooplankton were lower than those in coastal zooplankton collected vicinity to the nuclear power plant, also lower than those in the Baltic Sea after the Chernobyl accident, and comparable to those in the Adriatic and Mediterranean Seas after the Chernobyl accident.

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