Effects of variable daily light integrals and elevated CO2 on the adult and juvenile performance of two Acropora corals

Reef-building corals are subject to multi-day periods of reduced light and progressive ocean acidification. We experimentally assessed how adult and early post-settlement Acropora tenuis and A. hyacinthus corals responded to contrasting daily light integrals (DLI) and to multi-day variability in DLI, and whether contrasting DLIs altered the effects of ocean acidification. Four light treatments—three with stable DLIs (12.6, 7.6, 2.5 mol photons m −2 d −1 ) and one with variable DLI that averaged 7.6 mol photons m −2 d −1 were fully crossed with two levels of pCO 2 (400 and 900 ppm) in a 63-day aquarium experiment. Adult coral growth and protein content declined as average DLI declined, regardless of whether DLI was stable or variable. In both species, photoacclimation was insufficient to compensate for low DLI, although both effective (φ PSII ) and maximum (F v /F m ) quantum yields of photosystem two varied by