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.

Effects of diurnally oscillating pCO2 on the calcification and survival of coral recruits.

Manipulative studies have demonstrated that ocean acidification (OA) is a threat to coral reefs, yet no experiments have employed diurnal variations in pCO(2) that are ecologically relevant to many shallow reefs. Two experiments were conducted to test the response of coral recruits (less than 6 days old) to diurnally oscillating pCO(2); one exposing recruits for 3 days to ambient (440 µatm), high (663 µatm) and diurnally oscillating pCO(2) on a natural phase (420-596 µatm), and another exposing recruits for 6 days to ambient (456 µatm), high (837 µatm) and diurnally oscillating pCO(2) on either a natural or a reverse phase (448-845 µatm). In experiment I, recruits exposed to natural-phased diurnally oscillating pCO(2) grew 6-19% larger than those in ambient or high pCO(2). In experiment II, recruits in both high and natural-phased diurnally oscillating pCO(2) grew 16 per cent larger than those at ambient pCO(2), and this was accompanied by 13-18% higher survivorship; the stimulatory effect on growth of oscillatory pCO(2) was diminished by administering high pCO(2) during the day (i.e. reverse-phased). These results demonstrate that coral recruits can benefit from ecologically relevant fluctuations in pCO(2) and we hypothesize that the mechanism underlying this response is highly pCO(2)-mediated, night-time storage of dissolved inorganic carbon that fuels daytime calcification.