Can community structure and trophic level structure explain the variation in ¹³⁷Cs concentrations in zooplankton off Fukushima?. Results from the 2018 and 2020 investigations

A large amount of radionuclides was released from the TEPCO Fukushima Daiichi Nuclear Power Station (FDNPS), especially cesium (¹³⁷Cs and ¹³⁴Cs) and was diffused into the global ocean. After the accident, many studies on Cs concentrations in aquatic organisms have been conducted, however, many of them do not take community and/or trophic-level structures into account, especially for the organisms in the lower trophic levels. The reported decrease rate of Cs concentration in zooplankton is characterized by lower and more variable than that in seawater, however the reasons for this are not clear. We examined the relationship between ¹³⁷Cs concentrations in zooplankton and community structure and trophic level structure based on surveys in 2018 and 2020 in order to clarify the effect of the community and trophic level structures on ¹³⁷Cs concentrations in zooplankton. The community structure analysis showed that the zooplankton during the research periods was divided into nine communities based on 82% the Bray-Curtis similarity index. In both years, the community at st. NP0, which was closest to the FDNPS and had the highest ¹³⁷Cs concentrations, differed remarkably from the other stations. Comparison of ¹³⁷Cs concentrations in each community showed that ¹³⁷Cs concentrations were relatively high in communities with a high proportion of filter feeders, such as appendicularians, doliolids, and cladocerans. The relationship between ¹³⁷Cs concentration in zooplankton and δ¹⁵N values, an indicator of trophic level showed a positive correlation (Spearman's rank correlation, p < 0.05). This suggests that the predator-prey relationship (trophic structure) associated with zooplankton influences ¹³⁷Cs concentrations. On the other hand, there was no significant relationship between concentration ratio (CR) and δ¹⁵N values, suggesting that not only the predator-prey relationship but also the ¹³⁷Cs concentration in seawater may affect the ¹³⁷Cs concentration in bulk zooplankton. In conclusion, our study indicates that the ¹³⁷Cs concentration in zooplankton varies depending on the community structure and predator-prey relationship, while being also affected by the ¹³⁷Cs concentration in seawater. On the other hand, this study does not consider possible contamination of cesium-bearing microparticles (CsMP), which might lead to overestimation of ¹³⁷Cs concentrations in zooplankton. In addition, ctenophores, which occasionally appear in coastal areas, may also be underestimated because they cannot maintain their shape in fixed samples. For the better understanding of the ¹³⁷Cs dynamics in the marine ecosystems, future studies should be addressed these points. (author)