Testing the direct effect of CO sub(2) concentration on a bloom of the coccolithophorid Emiliania huxleyi in mesocosm experiments

We studied the direct effects of CO sub(2) and related changes in seawater carbonate chemistry on marine planktonic organisms in a mesocosm experiment. In nine outdoor enclosures ( similar to 11 m super(3) each), the partial pressure of CO sub(2) (pCO sub(2)) in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low ( similar to 190 parts per million by volume (ppmV) CO sub(2)), present ( similar to 410 ppmV CO sub(2)), and high ( similar to 710 ppmV CO sub(2)) pCO sub(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 sub(2) treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO sub(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 sub(2). This broad influence of CO sub(2) on the E. huxleyi bloom suggests that changes in CO sub(2) concentration directly affect cell physiology with likely effects on the marine biogeochemistry.