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

• Main Authors: Wulff, Angela, Karlberg, Maria, Olofsson, Malin, Torstensson, Anders, Riemann, Lasse, Steinhoff, Franciska, Mohlin, Malin, Ekstrand, Nina, Chierici, Melissa
• Format: Text
• Language: English
• Published: 2018
• Subjects: Ocean acidification
• Online Access: https://doi.org/10.5194/bg-2016-383; https://www.biogeosciences-discuss.net/bg-2016-383/

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 CO 2 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 CO 2 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 p CO 2 showed only minor effects on total biovolumes of phytoplankton and abundances of heterotrophic bacteria. No interaction effects of salinity and p CO 2 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 CO 2 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 p CO 2 , and considering the Baltic Proper, we do not expect any dramatic effects of increased p CO 2 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 p CO 2 and reduced salinity levels on the Baltic microplanktonic community.