The role of light in mediating the effects of ocean acidification on coral calcification

We tested the effect of light and P_CO2 on the calcification and survival of Pocillopora damicornis recruits settled from larvae released in southern Taiwan. In March 2011, recruits were incubated at 31,41,70,122 and 226μmol photons m^-2s^-1 under ambient (493μatm) and high P_CO2 (878μatm). After 5days, calcification was measured gravimetrically and survivorship estimated as the number of living recruits. Calcification was affected by the interaction of P_CO2 with light, and at 493 μatm P_CO2 the response to light intensity resembled a positive parabola. At 878μatm P_CO2, the effect of light on calcification differed from that observed at 493 μatm P_CO2, with the result that there were large differences in calcification between 493 μatm and 878 μatm P_CO2 at intermediate light intensities (ca. 70 μmol photons m^-2s^-1), but similar rates of calcification at the highest and lowest light intensities. Survivorship was affected by light and P_CO2, and was highest at 122μmol photons m^-2s^-1 in both P_CO2 treatments, but was unrelated to calcification. In June 2012 the experiment was repeated, and again the results suggested that exposure to high P_CO2 decreased calcification of P. damicornis recruits at intermediate light intensities, but not at lower or higher intensities. Together, our findings demonstrate that the effect of P_CO2 on coral recruits can be light dependent, with inhibitory effects of high P_CO2 on calcification at intermediate light intensities that disappear at both higher and lower light intensities.