Benthic food web structures as an explanation for prolonged ecological half-life of 137 Cs in flatfish species in the Fukushima coastal area

After the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP), Japan, in March 2011, Cs in demersal fish had, between 2011 and 2015, a prolonged ecological half-life when compared to pelagic fish. Using stable isotope mixing models combined with gut content analysis and Cs activity concentrations, this study investigated the hypothesis that an unexplored food web structure could be a contributing factor explaining the ecological half-life of Cs in benthic flatfish. Benthic invertebrates and demersal fish species sampled in 2015 still showed Cs activity concentrations higher than pre-accident. The mixing models of stable N and C isotopes and gut content analysis identified deposit, suspension and filter feeders to be the main flatfish food items in the benthos. There was a significant correlation between Cs activity concentrations in specific flatfish species and benthos, and between Cs activity concentrations in benthos and surface sediment. The results of this study partially explained the Cs activity concentrations found in the analysed demersal fish, suggesting that the benthos can be a continuous source of Cs for the demersal fish during this period of time. Extending monitoring programmes to include invertebrates that are not food species for humans would greatly improve our ability to understand the role of trophic transfer pathways and take appropriate management actions.