1.Rising atmospheric CO2 concentrations might affect the primary production and community composition of freshwater ecosystems. 2.We investigated these potential effects in laboratory mesocosms (Limnotrons), using monoculture experiments and competition experiments with the green alga Scenedesmus obliquus and the cyanobacterium Synechocystis sp. PCC6803. The Limnotrons were sparged with ambient air (controls, 380 parts per million volume (ppmv) CO2), moderately elevated CO2 levels (3000 ppmv CO2) or highly elevated CO2 levels (18 800 ppmv CO2). 3.Growth at ambient air led to the depletion of dissolved CO2 during algal bloom development and hence a high pH. In contrast, growth at elevated CO2 levels resulted in high concentrations of dissolved CO2 and dissolved inorganic carbon (DIC), lower pH and low concentrations of nitrate and soluble reactive phosphorus. Elevated CO2 levels did not have a significant effect on the biomass of the algal species, but shifted their elemental composition towards higher carbon-to-nutrient ratios. 4.Competition experiments at ambient air were driven mainly by competition for inorganic carbon. In this case, the cyanobacterium Synechocystis was displaced by the green alga Scenedesmus. Elevated CO2 alleviated the community from carbon limitation, which shifted the species interactions towards competition for nitrogen and phosphorus, and resulted in coexistence of the two species. Hence, our findings do not support the hypothesis that cyanobacteria are generally better competitors for inorganic carbon than green algae. 5.In conclusion, our results show that elevated CO2 levels may cause major changes in freshwater ecosystems, including a reduction in pH, shifts in resource limitation patterns, and changes in the ecological stoichiometry and species composition of phytoplankton communities.