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We studied the direct effects of CO₂ and related changes in seawater carbonate chemistry on marine planktonic organisms in a mesocosm experiment. In nine outdoor enclosures (~11 m³ each), the partial pressure of CO₂ (pCO₂) in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (~190 parts per million by volume (ppmV) CO₂), present (~410 ppmV CO₂), and high (~710 ppmV CO₂) pCO₂ 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 CO₂ treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO₂‐related effects. Normalization of production rates within each …