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494Engel et al. AbstractWe studied the direct effects of CO 2 and related changes in seawater carbonate chemistry on marine planktonic organisms in a mesocosm experiment. In nine outdoor enclosures (ϳ11 m 3 each), the partial pressure of CO 2 (pCO 2 ) in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (ϳ190 parts per million by volume (ppmV) CO 2 ), present (ϳ410 ppmV CO 2 ), and high (ϳ710 ppmV CO 2 ) pCO 2 conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyi occurred simultaneously in all of the nine mesocosms; it was monitored over a 19-day period. The three CO 2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO 2 -related effects. Normalization of production rates within each treatment, however, indicated that the net specific growth rate of E. huxleyi, the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO 2 . This broad influence of CO 2 on the E. huxleyi bloom suggests that changes in CO 2 concentration directly affect cell physiology with likely effects on the marine biogeochemistry.