Effects of land-rise on the development of a coastal ecosystem of the Baltic Sea and its implications for the long-term fate of 14C discharges

Due to the long half-lives of many radionuclides, safety assessments of nuclear waste facilities often need to consider the potential fate of radionuclides discharged to the environment in the future. In this study we explored the environmental fate of a hypothetical ¹⁴C release from a nuclear waste repository to a Baltic Sea bay in 2000 years. This was accomplished by connecting spatially linked biomasses and metabolic rates from a carbon flow model of the ecosystem at the site today to predicted changes in geomorphology and water exchange regimes. The employed extrapolation method was selected as shoreline displacement due to land-rise and sea level changes is the main process that affects the development of the coastal ecosystem around the repository in the coming 10 000 years. The modelling results indicate that the ecosystem will go through changes in several ecosystem properties in the coming 2000-year period, e.g. a decreased rate of primary production and changed feeding preferences of the fish community. Also, a decreased total biomass is expected and an ecosystem change altering the balance between producers and consumers towards a dominance of benthic plants. The changes of the ecosystem structure and carbon dynamics will also influence the potential fate of future discharges of ¹⁴C. We estimated an up to 1000 times higher ¹⁴C concentrations in biota compared to today. However, due to radioactive decay and reduced total biomass in the receiving ecosystem, the proportion of accumulated radionuclides is expected to decrease. Although the modelling approach used in this study is associated with several sources of uncertainty, it provides a way to both qualitatively and quantitatively assess likely effects of future discharges of contaminants.