Description: Species in the brackish and estuarine ecosystems will experience multiple changes in hydrographic variables due to ongoing climate change and nutrient loads. Here, we investigate how a glacial relict species (Saduria entomon), having relatively cold, low salinity biogeographic origin, could be affected by the combined scenarios of climate change and eutrophication. It is an important prey for higher trophic-level species such as cod, and a predator of other benthic animals. We constructed habitat distribution models based occurrence and density of this species across the entire Baltic and estimated the relative importance of different driving variables. We then used two regional coupled ocean-biogeochemical models to investigate the combined impacts of two future climate change and nutrient loads scenarios on its spatial distribution in 2070-2100. According to the scenarios, the Baltic Sea will become warmer and fresher. Our results show that expected changes in salinity and temperature outrank those due to two nutrient-load scenarios (Baltic Sea Action Plan and business as usual) in their effect on S. entomon distribution. The results are relatively similar when using different models with the same scenarios, thereby increasing the confidence of projections. Overall, our models predict a net increase (and local declines) of suitable habitat area, total abundance and biomass for this species, which is probably facilitated by strong osmoregulation ability and tolerance to temperature changes. We emphasize the necessity of considering multiple hydrographic variables when estimating climate change impacts on species living in brackish and estuarine systems.

Description: Eutrophication and climate change will affect habitats of species and more generally, the structure and functioning of ecosystems. We used a three‐dimensional, coupled hydrodynamic‐biogeochemical model to investigate potential future changes in size and location of potential habitats of marine species during the 21st century in a large, eutrophicated brackish sea (the Baltic Sea, northern Europe). We conducted scenario projections under the combined impact of nutrient load and climate change. Possible future changes of the eutrophication state of this sea were also assessed through two policy‐relevant indicators. The results imply a physiologically more stressful environment for marine species by the end of the 21st century: volumes of higher salinity water become more hypoxic/anoxic and the volumes of low salinity, oxic water increase. For example, these results impact and reduce cod reproductive habitats. The decrease is mainly climate change induced in the Baltic basins less directly influenced by inflows of saline, oxic water to the Baltic Sea (E Gotland and Gdansk Basins). In basins more directly influenced by such inflows (Arkona and Bornholm Basins), the combined effect from climate change and nutrient loads is of importance. The results for the eutrophication state indicators clearly indicate a more eutrophic sea than at present without a rigorous nutrient reduction policy, that is, the necessity to implement the Baltic Sea Action Plan. The multidisciplinary, multiscenario assessment strategy presented here provides a useful concept for the evaluation of impacts from cumulative stresses of changing climate and socioeconomic pressures on future eutrophication indicators and habitats of marine species.