4:45 PM - 5:00 PM
[MAG38-24] Temporal variation of ^{137}Cs in zooplankton and its primary factor in the waters off Fukushima and nearby prefectures
Keywords:Fukushima Dai-ichi Nuclear Power Plant accident, ^{137}Cs, Zooplankton, community composition
The East Japan earthquake on 11 March 2011 and the ensuing tsunami resulted in the release of large amounts of radionuclides from the accident of Fukushima Dai-ichi Nuclear Power Plant (FDNPP) into the surrounding environment. Cesium has similar chemical properties with potassium, which is the essential element for living organisms, and is easy to be incorporated into living organisms. The half-life of radioactive cesium (134Cs and 137Cs) is 2 and 30 years, respectively. For the relatively long half-life of 137Cs compared with 134Cs, continuous monitoring of the levels of 137Cs contamination is indispensable.
Zooplankton play a key role in marine biogeochemical cycle as secondary producers in the marine food web because they are major food for fishes and organisms of higher trophic levels. Therefore it is important to have information on their 137Cs concentration and behavior of 137Cs between zooplankton and surrounding environment such as seawater, surface sediment, and suspended marine particle.
Methods
In this study, zooplankton samples were collected at seven to eight sampling locations in the waters off Fukushima and nearby prefectures during May 2012 to January 2014. The 137Cs concentrations in the samples were measured with Ge detectors and the compositions of zooplankton species were analyzed by microscopic observation. The zooplankton samples were obtained by horizontal towing at a depth range of 0-80 m using a large ring net. Seawater and sediment samples were also collected at the corresponding locations with zooplankton samples.
Results
Concentrations of 137Cs in zooplankton ranged from 0.26 to 184 (Bq/kg-dry) during the sampling period (May 2012-January 2014). The concentration peaks did not appear concurrently at each station. In January 2014, concentrations of 137Cs in zooplankton ranged from 6.59-40.3 (Bq/kg-dry) and they were still one or two orders of magnitude larger than those detected before the accident of FDNPP (0.09-0.4 Bq/kg-dry, Kaeriyama et al., 2008). The taxonomic composition varied seasonally and geographically but Maxillopoda were generally dominant throughout the study.
137Cs in zooplankton are thought to be derived from surrounding environment such as seawater, surface sediments, suspended particle, and food. Since the 137Cs concentration in surface sediments has not decreased so fast as that in seawater with time, 137Cs in surface sediments might be incorporated into zooplankton by resuspension of surface sediments. Assuming that aluminum in zooplankton is only derived from surface sediments, the contributions of surface sediments to 137Cs in zooplankton were estimated as 10%. 137Cs in zooplankton in unit volume of seawater (μBq/m3) were not correlated with both of 137Cs in ambient seawater (Bq/L) and zooplankton biomass (mg-dry/m3). On the other hand, the timings of high relative abundance of Appendiculata, Osteichthyes and phytoplankton corresponded to the timings of relatively high concentrations of 137Cs in zooplankton. Therefore it is implied that variability in 137Cs concentrations in zooplankton attributed to the composition of plankton species rather than concentration of 137Cs in surrounding environment and plankton biomass.
Acknowledgements
This work was carried out as part of a research project contact with the secretariat of the nuclear regulation authority.
Reference
Kaeriyama, H., Watabe, T., and Kusakabe, M. (2008). 137Cs concentration in zooplankton and its relation to taxonomic composition in the western North Pacific Ocean. Journal of environmental radioactivity, 99(12), 1838-1845.
Zooplankton play a key role in marine biogeochemical cycle as secondary producers in the marine food web because they are major food for fishes and organisms of higher trophic levels. Therefore it is important to have information on their 137Cs concentration and behavior of 137Cs between zooplankton and surrounding environment such as seawater, surface sediment, and suspended marine particle.
Methods
In this study, zooplankton samples were collected at seven to eight sampling locations in the waters off Fukushima and nearby prefectures during May 2012 to January 2014. The 137Cs concentrations in the samples were measured with Ge detectors and the compositions of zooplankton species were analyzed by microscopic observation. The zooplankton samples were obtained by horizontal towing at a depth range of 0-80 m using a large ring net. Seawater and sediment samples were also collected at the corresponding locations with zooplankton samples.
Results
Concentrations of 137Cs in zooplankton ranged from 0.26 to 184 (Bq/kg-dry) during the sampling period (May 2012-January 2014). The concentration peaks did not appear concurrently at each station. In January 2014, concentrations of 137Cs in zooplankton ranged from 6.59-40.3 (Bq/kg-dry) and they were still one or two orders of magnitude larger than those detected before the accident of FDNPP (0.09-0.4 Bq/kg-dry, Kaeriyama et al., 2008). The taxonomic composition varied seasonally and geographically but Maxillopoda were generally dominant throughout the study.
137Cs in zooplankton are thought to be derived from surrounding environment such as seawater, surface sediments, suspended particle, and food. Since the 137Cs concentration in surface sediments has not decreased so fast as that in seawater with time, 137Cs in surface sediments might be incorporated into zooplankton by resuspension of surface sediments. Assuming that aluminum in zooplankton is only derived from surface sediments, the contributions of surface sediments to 137Cs in zooplankton were estimated as 10%. 137Cs in zooplankton in unit volume of seawater (μBq/m3) were not correlated with both of 137Cs in ambient seawater (Bq/L) and zooplankton biomass (mg-dry/m3). On the other hand, the timings of high relative abundance of Appendiculata, Osteichthyes and phytoplankton corresponded to the timings of relatively high concentrations of 137Cs in zooplankton. Therefore it is implied that variability in 137Cs concentrations in zooplankton attributed to the composition of plankton species rather than concentration of 137Cs in surrounding environment and plankton biomass.
Acknowledgements
This work was carried out as part of a research project contact with the secretariat of the nuclear regulation authority.
Reference
Kaeriyama, H., Watabe, T., and Kusakabe, M. (2008). 137Cs concentration in zooplankton and its relation to taxonomic composition in the western North Pacific Ocean. Journal of environmental radioactivity, 99(12), 1838-1845.