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

www.nature.com/scientificreports OPEN received: 28 July 2016 accepted: 29 November 2016 Published: 04 January 2017 Temporal variation of cesium isotope concentrations and atom ratios in zooplankton in the Pacific off the east coast of Japan Takahito Ikenoue1, Hyoe Takata1, Masashi Kusakabe1, Natsumi Kudo1, Kazuyuki Hasegawa1 & Takashi Ishimaru2,3 After the Fukushima Daiichi Nuclear Power Plant accident in March 2011, concentrations of cesium isotopes (133Cs, 134Cs, and 137Cs) 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 137 Cs concentration peaks in zooplankton. In addition, the atom ratio of 137Cs/133Cs 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 137Cs 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 137Cs concentrations in zooplankton as well as the higher ratio of 137Cs/133Cs in zooplankton than in seawater. The Great East Japan Earthquake occurred on 11 March 2011, and the ensuing tsunami resulted in the release of large amounts of radionuclides from the Fukushima Daiichi Nuclear Power Plant (FDNPP) into the atmosphere and ocean1,2. The main radionuclides discharged were 131I, 134Cs, and 137Cs, as reported by the Tokyo Electric Power Company (TEPCO)3. The half-lives of radiocesium, 134Cs and 137Cs, are 2.07 and 30.17 years, respectively. Due to their longer half-lives than 131I (8.04 days), continuous monitoring of the levels of radiocesium contamination is necessary to evaluate the impacts of these radionuclides on marine organisms, which is important for addressing risks to human health through consumption of fisheries resources. Zooplankton play an important role in the marine biogeochemical cycle as secondary producers in the food web, and are major food resource for fishes and organisms of higher trophic levels. Therefore, it is imperative to study the level and temporal variation of radiocesium in zooplankton in association with seawater, sediment, and suspended marine particles. The result will be of great help to predict the fate of radiocesium in marine ecosystem. After the FDNPP accident, elevated 137Cs concentrations in zooplankton were observed in the western North Pacific4–7. From May 2012 to January 2013, Takata et al.7 measured radiocesium concentrations in zooplankton collected at sampling locations identical to those used in this study; their findings suggested that the concentration of 137Cs in ambient seawater influenced the variations of 137Cs in zooplankton. However, Kaeriyama et al.6 carried out observations 10 months earlier than Takata et al.7 and noted that the temporal change of 137Cs in zooplankton was not synchronous with that of seawater. Based on these findings, they proposed a dynamic non-equilibrium model of 137Cs transfer between organisms and the surrounding seawater. The model described the progress of 137Cs contamination in zooplankton from the beginning of the FDNPP accident (dynamic non-equilibrium state) to the restoration phase (dynamic equilibrium state). In this study, we investigated the temporal variation of FDNPP-derived cesium isotopes in zooplankton collected from May 2012 to February 2015. We elucidated the factors controlling the changes in 137Cs concentration 1 Central Laboratory, Marine Ecology Research Institute, 300 Iwawada, Onjuku-machi, Isumi-gun, Chiba 299-5105, Japan. 2Marine Ecology Research Institute, Tohwa-Edogawbashi Bldg., 347 Yamabuki-cho, Shinjuku-ku, Tokyo 1620801, Japan. 3Department of Marine Science, Tokyo University of Marine Science and Technology, 5-7, Konan 4, Minato-ku, Tokyo 108-8477, Japan. Correspondence and requests for materials should be addressed to T.I. (email: ) Scientific Reports | 7:39874 | DOI: 10.1038/srep39874 1 www.nature.com/scientificreports/ 39°N -500 0 Miyagi B3 00 -10 38°N E5 E1 Fukushima G0 37°N 0 -200 FDNPP G4 Ibaraki -5 00 I1 0 00 J1 J3 -1 -3 0 00 -2000 36°N 0 35°N 140°E 141°E 142°E 143°E Figure 1. Map of the sampling locations of zooplankton, seawater, and surface sediment. Solid circles indicate sampling stations, and the star marks the Fukushima Daiichi Nuclear Power Plant (FDNPP). The Map is created online at http://sfb574.geomar.de/gmt-maps.html 8. in zooplankton by utilizing other relevant data such as stable cesium (133Cs) and aluminum concentrations, and the taxonomic composition of the zooplankton. Results and Discussion Abundance and taxonomic compositions of zooplankton. Zooplankton samples were collected at eight sampling sites in the Pacific off the east coast of Japan, where the radioactivity levels of seawater and surface sediments have been monitored under contract with the Japanese Ministry of Education, Culture, Sports, Science and Technology (2011–2013) and the Secretariat of the Nuclear Regulation Authority (2013-present) (Fig. 1). Seventy-nine samples were collected from May 2012 to February 2015. Table S1 summarizes the data on the plankton samples. Zooplankton biomass varied by two orders of magnitude, ranging from 1.1 to 562.8 mg-wet/m3, with water content ranging from 82% to 95%. The temporal variation of zooplankton abundance at the class level is shown in Table S1 and Fig. 2. Although the taxonomic composition varied seasonally and geographically, members of Maxillopoda were generally dominant throughout the study. Malacostraca increased from May 2012 to January 2013 at several stations. Appendiculata usually showed low relative abundance but increased in November or January, except at stn. G4. Branchiopoda increased in August 2013 at stns. B3, G0, and J3 and in August 2014 at stn, J1. Thaliacea increased in August 2013 at stns. B3, E1, and G0 and in August 2014 at stn. E1. The taxonomic compositions at stn. J1 in May 2012 (Osteichthyes dominant) and January 2013 (Appendiculata dominant) were especially different from those of the other stations. Osteichthyes was mainly composed of eggs of fishes in this study. It should be noted that thirteen of the samples were replete with microplankton, mainly chain-forming phytoplankton (see asterisks in Fig. 2). Temporal variation of radiocesium (134Cs and 137Cs) and the stable isotope (133Cs) in zooplankton. Temporal variation of radiocesium (134Cs and 137Cs) and the stable isotope (133Cs) in zooplankton are summarized in Fig. 3. Concentrations of 134Cs and 137Cs in zooplankton samples and relevant data for May 2013 to February 2015 are summarized in Table S2 (those for May 2012 to January 2013 were reported previously7). The concentrations of 137Cs in zooplankton var (...truncated)