Biogeosciences Discuss., 10, 6143-6170, 2013
www.biogeosciences-discuss.net/10/6143/2013/
doi:10.5194/bgd-10-6143-2013
© Author(s) 2013. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG.
Horizontal distribution of Fukushima-derived radiocesium in zooplankton in the northwestern Pacific Ocean
M. Kitamura1, Y. Kumamoto2, H. Kawakami3, E. C. Cruz4, and K. Fujikura1
1Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa, Japan
2Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa, Japan
3Mutsu Institute for Oceanography, Japan Agency for Marine-Earth Science and Technology, Mutsu, Aomori, Japan
4Pacific Islands Fisheries Science Center, NOAA Fisheries, Tamuning, Guam, USA

Abstract. The magnitude of the 9.0 Tohoku earthquake and the ensuing tsunami on 11 March 2011, inflicted heavy damage on the Fukushima Dai-ichi nuclear power plant (FNPP1). Fission products were emitted, falling over a broad range in the northern hemisphere, and water contaminated with radionuclides leaked into the ocean. In this study, we described the horizontal distribution of the Fukushima-derived radiocesium in zooplankton and in seawater in the western North Pacific Ocean (500–2100 km from the FNPP1) 10 months after the accident. 134Cs and 137Cs were detected in zooplankton and seawater from all the stations. Because of its short half-lives, 134Cs detected in our samples could only be derived from the FNPP1 accident. The highest 137Cs activity in zooplankton was same order of magnitude as that one month after the accident, and average activity was one or two orders of magnitude higher than 137Cs activities observed before the accident around Japan. Horizontally, the radiocesium activity concentrations in zooplankton were high at around 25° N while those in surface seawater were high at around the transition area between the Kuroshio and the Oyashio Currents (36–40° N). We observed subsurface radiocesium maxima in density range of the North Pacific Subtropical Mode Water and occurrence of many diel vertical migratory zooplanktons. These suggested that the high activity concentrations in the subtropical zooplankton at around 25° N were connected to the subsurface radiocesium and active vertical migration of zooplankton. However, the high activity concentrations of radiocesium in subsurface seawater did not necessarily follow the higher radiocesium activity in zooplankton. Biological characteristics of zooplankton community possibly influenced how large was contamination of radiocesium in the community but it is still unknown what kind of biological factors were important.

Citation: Kitamura, M., Kumamoto, Y., Kawakami, H., Cruz, E. C., and Fujikura, K.: Horizontal distribution of Fukushima-derived radiocesium in zooplankton in the northwestern Pacific Ocean, Biogeosciences Discuss., 10, 6143-6170, doi:10.5194/bgd-10-6143-2013, 2013.
 
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