In this study we apply the model ERGOM to simulate the consequences of Climate Change as well as the combination of Climate Change with nutrient load reductions according to the Baltic Sea Action Plan (BSAP) on the Baltic Sea ecosystem.
According to the simulations, Climate Change will cause an increase of the water temperature up to 3 K and a salinity decrease of 1.5 PSU until 2100. However, the implementation of the BSAP will have much stronger effects on the ecosystem. The model suggests that the western Baltic Sea will shift from a nitrogen (N) towards a phosphorus (P) limited system. As a consequence, N-fixation will strongly decrease. The same applies to nutrient-concentrations in winter, denitrification as well as detritus and chlorophyll concentrations in summer. The availability of N in summer, the Secchi depth and the oxygen saturation will increase. Our simulations suggest that the full implementation of the BSAP will cause imbalances in the Baltic Sea over decades before a new system state will be reached. Our results indicate that the Secchi depth alone is not a suitable indicator for the state of eutrophication. Concerning the Water Framework Directive (WFD), Climate Change might alter gradients and concentrations, e.g. of salinity, and in return this might require a re-definition of the WFD typology or a spatial shift of the surface water types, e.g. in Germany. The western Baltic Sea is strongly controlled by regional nutrient loads. Climate Change has only a limited effect on loads in the western Baltic. A re-definition of reference conditions and a good status because of Climate Change seems not necessary.