Effects of seawater pCO2 and temperature on calcification and productivity in the coral genus Porites spp.: an exploration of potential interaction mechanisms

Understanding how rising seawater pCO(2) and temperatures impact coral aragonite accretion is essential for predicting the future of reef ecosystems. Here, we report 2 long-term (10-11 month) studies assessing the effects of temperature (25 and 28 degrees C) and both high and low seawater pCO(2) (180-750 mu atm) on the calcification, photosynthesis and respiration of individual massive Porites spp. genotypes. Calcification rates were highly variable between genotypes, but high seawater pCO(2) reduced calcification significantly in 4 of 7 genotypes cultured at 25 degrees C but in only 1 of 4 genotypes cultured at 28 degrees C. Increasing seawater temperature enhanced calcification in almost all corals, but the magnitude of this effect was seawater pCO(2) dependent. The 3 degrees C temperature increase enhanced calcification rate on average by 3% at 180 mu atm, by 35% at 260 mu atm and by > 300% at 750 mu atm. The rate increase at high seawater pCO(2) exceeds that observed in inorganic aragonites. Responses of gross/net photosynthesis and respiration to temperature and seawater pCO(2) varied between genotypes, but rates of all these processes were reduced at the higher seawater temperature. Increases in seawater temperature, below the thermal stress threshold, may mitigate against ocean acidification in this coral genus, but this moderation is not mediated by an increase in net photosynthesis. The response of coral calcification to temperature cannot be explained by symbiont productivity or by thermodynamic and kinetic influences on aragonite formation.