The response of natural Baltic Sea spring plankton communities topredicted higher temperature and increased seawater acidification was studied in an indoor-mesocosm experiment. Plankton communities collected in a coastal area of the Baltic Sea (Kalmar Sound) were incubated for 20 days in the following conditions of: normal pH and temperature (pH=7.52, 3°C; control condition); lower pH (pH=7.14,3°C); high temperature (pH=7.52, 6°C) and lower pH-high temperature,(pH=7.14, 6 °C). Results showed that phytoplankton biomass (expressed as chlorophyll a), growth rates, plankton cell densities and community composition were significantly influenced by higher temperature, lower pH and to a greater extent subjected to both factors in combination. At higher temperature, phytoplankton biomass, particulate organic carbon(POC) and growth rates in addition to copepod densities were significantly enhanced. Highest bacteriae and heterotrophic nanoflagellates densities were observed in the higher temperature and lower pH treatment. Furthermore, the highest total phytoplankton and plankton communities diversity were found in this treatment as well. Increase in temperature and acidification accelerated the spring bloom by ca. 1 day °C-1. The phytoplankton community shifted from a dominance of Skeletonema costatum, in favor of haptophytes and dinoflagellates; and from dominance of the ciliate Myrionecta rubra in favor of tintiniids and oligotrichids. Our results suggest that the concomitant increased temperature and acidification of the Baltic Sea will increase the spring bloom biomass, and induce an early appearance of phytoplankton species typical of summer, thus decreasing the dominance of diatoms during the spring bloom. Decreased ciliates and copepod abundances will probably lead to sinking of higher amount phytoplankton biomass to deep water layers, expanding the area of oxygen depletion. If that is the case, the extra-incorporated carbon will not be channeled up the food chain.

Baltic Sea summer phytoplankton communities response to increased temperature, acidification and its combination, at levels predicted for the year 2100; were studied in a mesocosm experiment. The plankton communities were collected in a coastal area of the Baltic Proper andincubated for 12 days at the following conditions of: present pH and temperature (16°C and pH 8.1, present conditions), lower pH-present temperature (16°C and pH 7.8), higher temperature-present pH (18°C and pH 8.1) and lower pH-higher temperature (18°C and pH 7.7). The nutrient depleted-seawater containing the natural plankton communities were spiked daily with 0.12 μM K₃PO₄ and 1.25 μM NaNO₃. Phytoplankton biomass (chlorophyll a), growth rates, phytoplankton cell densities and diversity were measured through out the experiment. Phytoplankton biomass, growth rates and cell densities were significantly influenced by lower pH, higher temperature and at a greater extent when these two drivers were tested in combination. Higher temperature alone, lead to an acceleration of the summer bloom by ca. 1 days °C-1. Particulate organic carbon (POC) and particulate C:N and C:P atomic ratios were higher in all treatments than in the control condition at the end of the experiment. Higher temperatures increased phytoplankton biomass above the present temperature treatments. Shifts in phytoplankton composition were observed in all treatments. Diatoms were stimulated in the present condition, picocyanobacteriae in the lower pH-present temperature treatment and cyanobacteriae in the higher temperature treatments, including the filamentous, toxin-producer Nodularia spumigena. Highestdiversities were found in the present conditions and lower pH-higher temperature treatment, whereas lower pH alone lead to the lowest diversity found in all treatments. If seawater temperature continues to rise and pH to decrease as predicted by global climate change models, shifts in the phytoplankton community will occur and the size of the summer cyanobacteriae bloom will be bigger than already is now. The ultimate consequence will be less available edible algae to sustain the trophic foodweb and a decrease in the quality of the marine habitat. Alsosedimentation of these blooms to the deeper waters will increase oxygen depletion.