Increased free CO2 and ocean acidification are among the consequences of anthropogenic carbon emissions. Responses of marine protists to increased levels of CO2 are highly species-specific, and this has been suggested to cause an alteration in plankton species composition, community functions and ultimately biogeochemical cycles. This study aims to test this by performing microcosm incubation experiments at present (pH 8.0) and at three lowered pH levels (pH 7.8, 7.6 and 6.0), corresponding to free CO2 concentrations of 24, 38, 58 & 610 µmol l-1, respectively. Results from two such experiments are reported, and measurements include microscopy counts of ~20 planktonic protist taxa, HPLC pigment analysis, FlowCAM analysis of cell-size spectra, photosynthetic activity and total POC and PON. Initial communities were flagellate (experiment 1) and dinoflagellate and ciliate (experiment 2) dominated, but at pH 8.0, 7.8 and 7.6 a diatom dominance developed during the first days in both experiments. Profound changes in all measured parameters were found as a result of the pH 6.0 treatment, but among the other three pH treatments significant differences were few; Karlodinium spp. was the only taxon to be affected significantly, and plankton group composition, cell sizes and photosynthetic activity all remained unaffected during the 14-day incubation periods. Thus, both of the investigated coastal plankton communities were unaffected by twenty-first century expected changes in pH and free CO2. This may be explained by the large seasonal, and even daily, changes in pH seen in productive marine ecosystems, and the corresponding need for algae to be pH-tolerant.