Response of the temperate coral Cladocora caespitosa to mid- and long-term exposure to pCO₂ and temperature levels projected for the year 2100 AD

Abstract. Atmospheric CO₂ partial pressure (pCO₂) is expected to increase to 700 μatm or more by the end of the present century. Anthropogenic CO₂ is absorbed by the oceans, leading to decreases in pH and the CaCO₃ saturation state (Ω) of the seawater. Elevated pCO₂ was shown to drastically decrease calcification rates in tropical zooxanthellate corals. Here we show, using the Mediterranean zooxanthellate coral Cladocora caespitosa, that an increase in pCO₂, in the range predicted for 2100, does not reduce its calcification rate. Therefore, the conventional belief that calcification rates will be affected by ocean acidification may not be widespread in temperate corals. Seasonal change in temperature is the predominant factor controlling photosynthesis, respiration, calcification and symbiont density. An increase in pCO₂, alone or in combination with elevated temperature, had no significant effect on photosynthesis, photosynthetic efficiency and calcification. The lack of sensitivity C. caespitosa to elevated pCO₂ might be due to its slow growth rates, which seem to be more dependent on temperature than on the saturation state of calcium carbonate in the range projected for the end of the century.