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Temperature and CO2 levels are projected to increase in the future, with consequences for carbon and nutrient cycling in brackish environments, such as the Baltic Sea. Moreover, filamentous cyanobacteria are predicted to be favored over other phytoplankton groups under these conditions. Under a 12-day outdoor experiment, we examined the effect on a natural phytoplankton spring bloom community of elevated temperature (from 1 degrees C to 4 degrees C) and elevated pCO(2) (from 390 to 970 mu atm). No effects of elevated pCO(2) or temperature were observed on phytoplankton biovolumes, but a significantly higher photosystem II activity was observed at elevated temperature after 9 days. In addition, three species of diazotrophic filamentous cyanobacteria were inoculated to test their competitive capacity under spring bloom conditions. The toxic cyanobacterium Nodularia spumigena exhibited an average specific growth rate of 0.10 d(-1) by the end of the experiment, indicating potential prevalence even during wintertime in the Baltic Sea. Generally, none of the inoculated cyanobacteria species were able to outcompete the natural phytoplankton species at temperatures <= 4 degrees C. No direct effects were found on heterotrophic bacteria. This study demonstrates the highly efficient resistance towards short-term (12 days) changes in abiotic factors by the natural Baltic Sea spring bloom community.