To investigate the effect of diel variation in pCO2 on coral calcification, branches of Acropora hyacinthus were collected in 2 habitats (upstream and downstream in a unidirectional flow) in a shallow back reef in Moorea, French Polynesia, where different diel amplitudes of pCO2 oscillation were expected. Corals were maintained for 6 wk under different pCO2 regimes (constant versus oscillatory), each delivered in 3 configurations: constant conditions of 400 µatm, 700 µatm, and 1000 µatm pCO2, or oscillatory conditions varying daily from 280 to 550 µatm, 550 to 1000 µatm, or 400 to 2000 µatm, with minima and maxima during the day and night, respectively. Calcification rates in all treatments tended to increase over time, and the interaction between Time and pCO2 regime (i.e. constant versus oscillating) was significant (or close to significant) for upstream corals due to higher calcification in oscillatory pCO2. A significant pCO2 regime effect was detected in the highest pCO2 for downstream corals, with higher calcification in the 400 to 2000 µatm oscillatory pCO2 treatment compared to the 1000 µatm constant pCO2 treatment. After 6 wk, calcification of A. hyacinthus was affected significantly by habitat, the pCO2 level, and the pCO2 regime. Calcification generally was reduced by high pCO2 and was ≥21% greater in 400 to 2000 µatm oscillatory pCO2 versus 1000 µatm constant pCO2 treatment. Increased calcification in the 400 to 2000 µatm oscillatory pCO2 treatment suggests that natural diel oscillations in pCO2 could play a role by reducing the locally negative effects of rising pCO2 associated with ocean acidification on coral calcification.
