In July 2007 free floating mesocosm with app. 60m³ of volume were deployed in the southern Baltic Proper for app. two weeks. A gradient of increasing pCO2 concentrations was adjusted starting with the present concentration of 312 µatm up to 1600µatm as highest concentration. The response of the autotrophic plankton community was recorded in terms of nitrogen fixation rates. Heterotrophic activity was calculated based on the uptake of radiolabeled Thymidine. There was no clear increase of nitrogen fixation rates with higher pCO2. It rather seemed that the colonial species >10µm suffered under decreasing pH while the size fraction <10µm did not show any significant response In contrast to that cell specific bacterial biomass production sharply decreased within the first 24 hours after acidification but showed increasing activity with increasing CO2 concentration for the rest of the experiment. . A daily cycle of N-fix rates was only visible in the mesocosm with no pCO2 changes which also had highest N-fixation rates of up to 1.4nmol N/L/h.Our results indicate decoupling of primary and secondary production under increasing CO2 concentrations.