Ecosystem model-based approach for modeling the dynamics of &amp;lt;sup&amp;gt;137&amp;lt;/sup&amp;gt;Cs transfer to marine plankton populations: application to the western North Pacific Ocean after the Fukushima nuclear power plant accident

M Belharet,C Estournel,S Charmasson

doi:10.5194/bg-13-499-2016

Abstract

Abstract. Huge amounts of radionuclides, especially 137Cs, were released into the western North Pacific Ocean after the Fukushima nuclear power plant (FNPP) accident that occurred on 11 March 2011, resulting in contamination of the marine biota. In this study we developed a radioecological model to estimate 137Cs concentrations in phytoplankton and zooplankton populations representing the lower levels of the pelagic trophic chain. We coupled this model to a lower trophic level ecosystem model and an ocean circulation model to take into account the site-specific environmental conditions in the area. The different radioecological parameters of the model were estimated by calibration, and a sensitivity analysis to parameter uncertainties was carried out, showing a high sensitivity of the model results, especially to the 137Cs concentration in seawater, to the rates of accumulation from water and to the radionuclide assimilation efficiency for zooplankton. The results of the 137Cs concentrations in planktonic populations simulated in this study were then validated through comparison with the data available in the region after the accident. The model results have shown that the maximum concentrations in plankton after the accident were about 2 to 4 orders of magnitude higher than those observed before the accident, depending on the distance from FNPP. Finally, the maximum 137Cs absorbed dose rate for phyto- and zooplankton populations was estimated to be about 5 × 10−2 µGy h−1, and was, therefore, lower than the predicted no-effect dose rate (PNEDR) value of 10 µGy h−1 defined in the ERICA assessment approach.

Highlights

Huge amounts of radionuclides, especially 137Cs, were released into the western North Pacific Ocean after the Fukushima nuclear power plant (FNPP) accident that occurred on 11 March 2011 (UNSCEAR, 2014).Plankton populations, which play a prominent role in the input of many pollutants into the aquatic food chain and are potentially important in the biogeochemical cycling of various radionuclides in the ocean (Fowler and Fisher, 2004), were contaminated by these releases.Data on 137Cs in phytoplankton are rare especially due to difficulties in sampling
We presented a modeling approach based on an ecosystem model to estimate the 137Cs activity in marine plankton populations following the Fukushima nuclear power plant (FNPP) accident, and to understand the effect of this accident on the different processes related to the radiocesium transfer in the planktonic trophic levels
This study showed that the maximum values of the 137Cs concentrations in phytoplankton and zooplankton populations were mainly reached 1 month after the accident and were about 2 to 4 orders of magnitude higher than those observed before the accident depending on the distance from FNPP

Summary

Introduction

Especially 137Cs, were released into the western North Pacific Ocean after the Fukushima nuclear power plant (FNPP) accident that occurred on 11 March 2011 (UNSCEAR, 2014).Plankton populations, which play a prominent role in the input of many pollutants into the aquatic food chain and are potentially important in the biogeochemical cycling of various radionuclides in the ocean (Fowler and Fisher, 2004), were contaminated by these releases.Data on 137Cs in phytoplankton are rare especially due to difficulties in sampling. Especially 137Cs, were released into the western North Pacific Ocean after the Fukushima nuclear power plant (FNPP) accident that occurred on 11 March 2011 (UNSCEAR, 2014). Recently Baumann et al (2015) reported 137Cs data on suspended matter rich in marine phytoplankton sampled in June 2011 off the Japanese coast (Buesseler et al, 2012) and suggested that phytoplankton could have been a substantial source of 137Cs for zooplankton after the Fukushima accident. Three months after the accident, Buesseler et al (2012) reported that the 137Cs concentrations in zooplankton located at 300–600 km from FNPP were 2 to 3 orders of magnitude higher than before the accident. Even 10 months after the accident, the 137Cs concentrations observed in zooplankton, at 600–2100 km away from FNPP, were still about 1 to 2 orders of magnitude higher than in the pre-accident period (Kitamura et al, 2013)

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