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

After the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP), Japan, in March 2011, 137Cs 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 137Cs activity concentrations, this study investigated the hypothesis that an unexplored food web structure could be a contributing factor explaining the ecological half-life of 137Cs in benthic flatfish. Benthic invertebrates and demersal fish species sampled in 2015 still showed 137Cs 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 137Cs activity concentrations in specific flatfish species and benthos, and between 137Cs activity concentrations in benthos and surface sediment. The results of this study partially explained the 137Cs activity concentrations found in the analysed demersal fish, suggesting that the benthos can be a continuous source of 137Cs 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.

Deoxygenation reduces growth rates and increases assimilation of essential trace metals in gilthead seabream (Sparus aurata).

The widespread decline in oceanic dissolved oxygen (DO), known as deoxygenation, is a threat to many marine ecosystems, and fish are considered one of the more vulnerable marine organisms. While food intake and growth rates in some fish can be reduced under hypoxic conditions (DO ~ 60 µmol kg-1), the dietary transfer of essential metals remains unclear. In this context, we investigated the influence of DO on the dietary acquisition of two essential metals (Zn and Mn) in the commercially important gilthead seabream (Sparus aurata) using radiotracer techniques. Fish were exposed to variable DO conditions (normoxia 100% DO, mild-hypoxia 60% DO, and hypoxia 30% DO), and fed a single radiolabeled food ration containing known activities of 54Mn and 65Zn. Depuration and assimilation mechanisms under these conditions were followed for 19 d. Based on whole body activity after the radio-feeding, food consumption tended to decrease with decreasing oxygen, which likely caused the significantly reduced growth (- 25%) observed at 30% DO after 19 d. While there was an apparent reduction in food consumption with decreasing DO, there was also significantly higher essential metal assimilation with hypoxic conditions. The proportion of 65Zn remaining was significantly higher (~60%) at both low DO levels after 24 h and 19 d while 54Mn was only significantly higher (27%) at the lowest DO after 19 d, revealing element specific effects. These results suggest that under hypoxic conditions, stressed teleost fish may allocate energy away from growth and towards other strategic processes that involve assimilation of essential metals.

Trophic Transfer of Radioactive Micronutrients in a Shallow Benthic Food Web.

The Baltic Sea is one of the most polluted seas in the world, with widespread eutrophication and radionuclide contamination. Using key species of the Baltic Sea, the effects of eutrophication on uptake and trophic transfer of the radioactive micronutrients commonly found in nuclear power plant effluents were investigated experimentally using the brown macroalgae Fucus vesiculosus and the grazers Idotea balthica and Theodoxus fluviatilis in a controlled environment. Rapid uptake of 54 Mn, 57 Co, and 65 Zn from water was observed in all biota; and eutrophication combined with grazing pressure strongly influenced the uptake in F. vesiculosus. Uptake of 54 Mn, 57 Co, and 65 Zn to I. balthica and T. fluviatilis grazing on F. vesiculosus were also observed. The results indicate that ecosystems could be open for further trophic transfer as radionuclides accumulate quickly in the producers and are transferred to primary consumers. Environ Toxicol Chem 2021;40:1694-1705. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.