Climate-driven change in a Baltic Sea summer microplanktonic community – desalination play a more important role than ocean acidification

Abstract. Scenario modeling suggests that the Baltic Sea, one of the largest brackish-water bodies in the world, could expect increased precipitation (decreased salinity) and increased concentration of atmospheric CO₂ over the next 100 years. These changes are expected to affect the microplanktonic food web, and thereby nutrient and carbon cycling, in a complex and possibly synergistic manner. In the Baltic Proper, the extensive summer blooms dominated by the filamentous cyanobacteria Aphanizomenon sp., Dolichospermum sp. and the toxic Nodularia spumigena, contribute up to 30 % of the yearly new nitrogen and carbon exported to the sediment. In a 12 days outdoor microcosm experiment, we tested the combined effects of decreased salinity (from 6 to 3) and increased CO₂ concentrations (380 and 960 µatm) on a natural summer microplanktonic community, focusing on diazotrophic filamentous cyanobacteria. Based on our results, the most important factor was salinity, and pCO₂ showed only minor effects on total biovolumes of phytoplankton and abundances of heterotrophic bacteria. No interaction effects of salinity and pCO₂ were found on any of the measured parameters. The biovolume of the toxic N. spumigena was negatively affected by salinity 3, and the treatment with salinity 3 and 960 µatm CO₂ resulted in increased biomass of the presumably non-toxic Dolichospermum sp. Biovolumes of ciliates, diatoms and dinoflagellates were lower in salinity 3. Thus, the lower salinity seemed more important than increased pCO₂, and considering the Baltic Proper, we do not expect any dramatic effects of increased pCO₂ in combination with decreased salinity on the microplanktonic food web. We believe that our study can add one piece to the complicated puzzle to reveal the combined effects of increased pCO₂ and reduced salinity levels on the Baltic microplanktonic community.