Diel pCO2 oscillations modulate the response of the coral Acropora hyacinthus to ocean acidification

To investigate the effect of diel variation in pCO 2 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 pCO 2 oscillation were expected. Corals were maintained for 6 wk under different pCO 2 regimes (constant versus oscillatory), each delivered in 3 configurations: constant conditions of 400 atm, 700 atm, and 1000 atm pCO 2 , or oscillatory conditions varying daily from 280 to 550 atm, 550 to 1000 atm, or 400 to 2000 atm, 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 pCO 2 regime (i.e. constant versus oscillating) was significant (or close to significant) for upstream corals due to higher calcification in oscillatory pCO 2 . A significant pCO 2 regime effect was detected in the highest pCO 2 for downstream corals, with higher calcification in the 400 to 2000 atm oscillatory pCO 2 treatment compared to the 1000 atm constant pCO 2 treatment. After 6 wk, calcification of A. hyacinthus was affected significantly by habitat, the pCO 2 level, and the pCO 2 regime. Calcification generally was reduced by high pCO 2 and was 21% greater in 400 to 2000 atm oscillatory pCO 2 versus 1000 atm constant pCO 2 treatment. Increased calcification in the 400 to 2000 atm oscillatory pCO 2 treatment suggests that natural diel oscillations in pCO 2 could play a role by reducing the locally negative effects of rising pCO 2 associated with ocean acidification on coral calcification.