The Earth’s climate is determined different processes occurring in the atmosphere, land and ocean.Anthropogenic activities (mainly combustion of fossil fuels) have increased since the 1950’s, andthus the concentration of greenhouse gases that are warming up the planet. Such events have andwill likely continue to have profound impact on ocean biological processes directly affecting thephysiology of marine organisms and population dynamics.This thesis deals with the effects of climate-induced changes in seawater temperature and pH(acidification) on the development and structure of Baltic Sea plankton communities. Mesocosmexperiments were set up to expose both spring and summer Baltic Sea plankton communities totemperature and pH levels predicted by the year 2100, both isolated and in combination.Experiments lasted for 20 and 12 experimental days, for spring and summer seasons, respectively.Parameters analyzed were phytoplankton biomass (Chlorophyll a) and growth rates (Chlorophyll aaccumulation) and phytoplankton, microzooplankton (ciliates and heterotrophic nanoflagellates),mesozooplankton (copepods) and bacteriae densities. The obtained results were later compared(ANOVA test) and diversity indexes were calculated for phytoplankton, ciliate and totalcommunities. Cluster analyses were done based on species/groups composition of the planktoncommunities.The highest phytoplankton (chlorophyll a) and total plankton (Particulate organic carbon-POC)biomass were found during both experiments in the higher temperature treatments. During spring,the phytoplankton biomass found in the present conditions treatment on the last day experimentalwas achieved 3 days earlier in the higher temperature treatmtents. Thus the peak of the springbloom was accelerated by c. 1 day °C-1. Lower pH treatments increased phytoplankton biomassabove the present conditions in the spring, but did not in summer. However, higher POC wereobserved in the lower pH treatment during both spring and summer. During the spring, thecombination of lower pH and higher temperature had a greater impact on the natural planktoncommunities than each isolated factor. Spring phytoplankton communities were initiallydominated by the diatom Skeletonema costatum, while in the summer cyanobacteriae dominated(although in both cases in very low densities). In the lower pH-higher temperature treatment fromthe spring, the phytoplankton community shifted and haptophytes and dinoflagellates becamemore representative, whereas in summer potentially toxic, filamentous cyanobacteriae (includingharmful, bloom-forming Nodularia spumigena) increased in numbers.In the lower pH and high temperature treatments of the spring experiment and in all treatments ofthe summer experiment, decreases in ciliate and nanoflagellate densities were correlated to highcopepod densities. Although the phytoplankton biomass was favored by higher temperature andacidification, a slight decrease of diatoms and a shift towards a community dominated by smallsized species was observed during both experiments. Predicted changes in seawater temperatureand pH will result in more carbon being incorporated into the planktonic food webs of the BalticSea, both in spring and summer, with shifts in species composition that will likely change thecarbon pathways. Further studies are needed to clarify the transfer efficiency of the incorporatedcarbon to higher trophic levels, and the resulting net productivity of the system in face of the new conditions.