Effects of variable daily light integrals and elevated CO.sub.2 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.sup.-2 d.sup.-1) and one with variable DLI that averaged 7.6 mol photons m.sup.-2 d.sup.-1 were fully crossed with two levels of pCO.sub.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 ([phi].sub.PSII) and maximum (F.sub.v/F.sub.m) quantum yields of photosystem two varied by < 5% between all stable DLI treatments. Under variable DLI, both species adjusted their [phi].sub.PSII on the day of change in DLI, whereas F.sub.v/F.sub.m remained relatively constant despite five-fold difference in DLI between days. Elevated CO.sub.2 increased protein content in adult A. tenuis at all DLIs, but otherwise had little effect on measured parameters. For juveniles, both species had reduced survival at low DLI due to overgrowth by Peyssonnelia algae, and A. tenuis growth was fastest at low DLI. Our study shows that the effects of multi-day periods of DLI reductions accumulate over time for corals, negatively affecting Acropora adult growth rates and juvenile survival, and hence slowing reef recovery after disturbance.