Impacts of changing climate on the non-indigenous invertebrates in the northern Baltic Sea by end of the twenty-first century

Biological invasions coupled with climate change drive changes in marine biodiversity. Warming climate and changes in hydrology may either enable or hinder the spread of non-indigenous species (NIS) and little is known about how climate change modifies the richness and impacts of NIS in specific sea areas. We calculated from climate change simulations (RCO-SCOBI model) the changes in summer time conditions which northern Baltic Sea may to go through by the end of the twenty-first century, e.g., 2–5 °C sea surface temperature rise and even up to 1.75 unit decrease in salinity. We reviewed the temperature and salinity tolerances (i.e., physiological tolerances and occurrence ranges in the field) of pelagic and benthic NIS established in—or with dispersal potential to—the northern Baltic Sea, and assessed how climate change will likely affect them. Our findings suggest a future decrease in barnacle larvae and an increase in Ponto-Caspian cladocerans in the pelagic community. In benthos, polychaetes, gastropods and decapods may become less abundant. By contrast, dreissenid bivalves, amphipods and mysids are expected to widen their distribution and increase in abundance in the coastal areas of the northern Baltic Sea. Potential salinity decrease acts as a major driver for NIS biogeography in the northern Baltic Sea, but temperature increase and extended summer season allow higher reproduction success in bivalves, zooplankton, amphipods and mysids. Successful NIS, i.e., coastal crustacean and bivalve species, pose a risk to native biota, as many of them have already demonstrated harmful effects in the Baltic Sea.