Warming, but not enhanced CO2 concentration, quantitatively and qualitatively affects phytoplankton biomass

We investigated the impacts of predicted ocean acidification and future warming on the quantity and nutritional quality of a natural phytoplankton autumn bloom in a mesocosm experiment. Since the effects of CO2-enrichment and temperature have usually been studied independently, we were also interested in the interactive effects of both aspects of climate change. Therefore, we used a factorial design with 2 temperature and 2 acidification levels in a mesocosm experiment with a Baltic Sea phytoplankton community. Our results show a significant time-dependent influence of warming on phytoplankton carbon, chlorophyll a, and particulate organic carbon. Phytoplankton carbon, for instance, decreased by more than half with increasing temperature at bloom time. Additionally, elemental carbon to phosphorus ratios (C:P) increased significantly, by approximately 5 to 8%, due to warming. Impacts of CO2 or synergetic effects of warming and acidification could not be detected. We suggest that stronger grazing pressure induced by temperature was responsible for the significant decline in phytoplankton biomass. Our results suggest that the biological effects of warming on Baltic Sea phytoplankton are considerable and will likely have fundamental consequences for trophic transfer in the pelagic food web.