Coral calcification response to ocean warming and acidification in the southern Great Barrier Reef

Abstract

The current unprecedented rate of increase in atmospheric carbon dioxide levels is resulting in rapid warming and acidification of the surface ocean. The effect that these changes have had and will continue to have on marine calcifiers is poorly constrained. Particularly, calcification on high latitude coral reefs which lie near the limit for reef growth could be expected to respond first to changes in ocean pH and temperature. Accordingly, this study aims to determine to what extent rising temperatures and increasing ocean acidity have and will affect coral calcification in the southern GBR. To address this aim, long-term multiple proxy records of temperature and seawater pH were measured along with corresponding calcification rates from four Porites corals for the period of 1834-2008. Seasonal coral proxy records reveal the significant physiological modification of temperature proxy signals and reef water chemistry during calcification. Reconstructions of SST based on Sr/Ca and O isotopes show suppressed variation compared to instrumental SST. This can be attributed to an attenuation of SST records in coral skeleton by the bio-smoothing effect of the living tissue layer, particularly during winter when coral growth slows. The seasonal reef water pH derived from coral B isotope is characterized by higher acidity in summer than winter with greater pH variations relative to the open ocean. This reflects changes in local reef water pH mainly due to a build-up carbon dioxide as a result of higher coral calcification and lower wind-driven flushing efficiency in summer. Long coral proxy records in this study provide direct evidence for significant ocean warming and acidification of reef waters in the southern GBR over the last 170 years. Ocean warming has occurred at a rate of 1.0 degree over the past 170 years, which is consistent with the global average temperature increase over the same period. Southern GBR reef waters have become 0.21 pH units more acidic over the same period, which is approximately a factor of two more rapid than the global ocean acidification rate. These long-term changes are overprinted with decadal-scale variability that is associated with ocean-atmosphere anomalies in the southern GBR and the southwest Pacific. Despite the significant increase in ocean acidity over the past 170 years, coral calcification has increased by 12% due to a concurrent increase in seawater temperature. An estimate of cumulative impact of the two competing influences on calcification shows that an increase in SST is a dominant factor that is controlling changes in coral calcification as atmospheric carbon dioxide level increases. Future coral calcification in the southern GBR, based on modeled trajectories of reef water pH and temperature changes under the IPCC emission scenarios, is projected to remain approximately constant or possibly undergo a slight decrease by the end of this century. This study provides evidence, subject to future emission scenarios, that ocean acidification has the potential to cause a sufficiently large decrease in seawater saturation state that may overwhelm the ability of corals to up-regulate pH in their calcifying fluid and kinetic enhancement of calcification due to ocean warming.