Determining food web transfer of radionuclides in marine benthic ecosystems

Abstract: Food web interactions are vital in any functioning ecosystem and facilitate transfer of energy and nutrients between trophic levels. Unfortunately, they also facilitate transfer of contaminants such as anthropogenic radionuclides found in effluents from nuclear power plants (NPPs). The recipients for most liquid NPP effluents are nearby benthic ecosystems. However, little is known of the fate of these types of radionuclides in benthic ecosystems, or how environmental factors influence these processes. In this thesis an array of methods were deployed to identify pathways by which anthropogenic radionuclides enter and transfer through different marine benthic ecosystems. Field sampling was done along the coast off the Fukushima Dai-ichi NPP and food web interactions were identified that helped to explain persisting radioactivity in benthic flatfish (Paper I). The effects of grazing and eutrophication on radionuclide transfer were studied using a Baltic Sea benthic cosm experiment (Paper II). Live counting of radioactivity in benthic fish after consuming clams contaminated with radioactive trace metals was done to identify uptake, transfer, and retention in a benthic ecosystem (Paper III). The effects of hypoxia on transfer and retention in a benthic-pelagic pathway was studied in similar way to Paper III by feeding pelagic fish under hypoxia with contaminated clams (Paper IV).The studies showed a species- and radionuclide-specific uptake and transfer of radionuclides (Paper I-III), an explanation that both trophic and abiotic transfer could prolong the radioactivity in benthic fish off Fukushima (Paper I), a radionuclide uptake into macroalgae affected positively by a combination of eutrophication and grazing (Paper II), and an increased retention of radionuclides in fish under hypoxic conditions (Paper III-IV). Together, these studies give tangible evidence of radionuclide uptake and transfer that are specific to species and radionuclide. Radionuclide fate is also affected by environmental stressors, such as eutrophication and hypoxia, and is therefore highly dependent on multiple factors. Consequently, local benthic ecosystems, as well as any present ecosystemic stressors, must be accounted for when planning and assessing risks in, for example, construction and operation of NPPs or in the event of possible accidental releases. This thesis provides an insight into methods that can be used to account for such ecological processes while still generating useful results that are applicable in risk management and modelling of NPP-derived radionuclides in benthic ecosystems.