Diel pCO sub(2) oscillations modulate the response of the coral Acropora hyacinthus to ocean acidification

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