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

We tested the effect of light and PCO2 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 226 μmol photons m(-2) s(-1) under ambient (493 μatm) and high PCO2 (878 μatm). After 5 days, calcification was measured gravimetrically and survivorship estimated as the number of living recruits. Calcification was affected by the interaction of PCO2 with light, and at 493 μatm PCO2 the response to light intensity resembled a positive parabola. At 878 μatm PCO2, the effect of light on calcification differed from that observed at 493 μatm PCO2, with the result that there were large differences in calcification between 493 μatm and 878 μatm PCO2 at intermediate light intensities (ca. 70 μmol photons m(-2) s(-1)), but similar rates of calcification at the highest and lowest light intensities. Survivorship was affected by light and PCO2, and was highest at 122 μmol photons m(-2) s(-1) in both PCO2 treatments, but was unrelated to calcification. In June 2012 the experiment was repeated, and again the results suggested that exposure to high PCO2 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 PCO2 on coral recruits can be light dependent, with inhibitory effects of high PCO2 on calcification at intermediate light intensities that disappear at both higher and lower light intensities.