Award: OCE-1220529

Coral reef ecosystems support the livelihoods of 500 million people worldwide, provide coastline protection worth billions of dollars, and provide habitat for 25% of all marine species. Coral reefs are built by organisms, mainly corals and coralline algae that produce calcium carbonate (CaCO3) skeletons. Healthy coral reefs produce more CaCO3 than they lose to waves, storms and bioerosion, a delicate balance that has been maintained for thousands of years. Anthropogenic climate change threatens this balance. Laboratory experiments show that ocean acidification, the decline in ocean pH caused by absorption of anthropogenic CO2, makes it more difficult for organisms to produce CaCO3 while making CaCO3 skeletons more susceptible to erosion and dissolution. We investigated the main factors controlling rates of CaCO3 production by corals and coral reef communities. First, we quantified rates of calcification and bioerosion in situ, by massive Porites corals across the natural pH, temperature and productivity gradient of the tropical Pacific basin. We found that coral calcification and bioerosion correlate strongly with water column primary productivity, represented by chlorophyll a concentrations. Corals in highly productive waters calcify faster, but they also erode faster. Both calcification and erosion exhibit sensitivity to the gradient in ocean pH. Calcification slows and bioerosion rates increase with decreasing pH. However, the influence of pH is weaker than the influence of primary productivity. Overall, rates of CaCO3 production by coral colonies is highest on reefs where productivity and pH are high; conversely, rates of bioerosion are highest when productivity is high and pH is low. Nevertheless today, Porites coral calcification rates still exceed bioerosion rates by a wide margin. We then measured Net Ecosystem Calcification (NEC) by three different reef communities to evaluate the main controls on net calcification on the reef scale. NEC is the balance between how much CaCO3 is produced by reef organisms and how much is dissolved away, over a period of several days. We replicated measurements of NEC by the same reef community in different weeks, different seasons and different years. We found large changes in NEC rates between different deployments at the same site, in the absence of any changes in seawater pH. Lowest NEC rates were recorded at the lowest pH reef on Palau. However, we did not find a correlation between pH and NEC across our three study sites. We compiled our NEC data together with those generated by other groups on other reefs around the world (n=31). We found no relationship between NEC and the pH of the source water feeding the reef. Together, these observations indicate that currently, ocean pH is not a good predictor of net CaCO3 production on coral reefs in nature. Key insights into the drivers of NEC come from our study on Dongsha Atoll, northern South China Sea, where we made hourly measurements of NEC and NEP (Net Ecosystem Productivity) on the 3-km wide reef flat, over two four-day deployments. Here, NEC rates were the highest ever recorded on a reef, anywhere, despite "normal" ocean pH ~8-8.1 values and relatively low coral cover of ~21%. However, hourly NEC and NEP on the reef flat were tightly correlated: when productivity was high, calcification was high too. One of our deployments coincided with a transient coral bleaching event caused by anomalously high water temperatures. During bleaching, NEP declined and NEC rates declined also, in the absence of any change in the pH of the open ocean source water to the reef. Our results highlight the important role of primary productivity for calcification by corals and coral reefs alike. While the mechanisms underlying this link are not yet clear, it is possible that productivity, by coral symbionts, benthic algae, and reef phytoplankton, modulate the pH of the environment in which calcification occurs, both within the coral itself and on the reef. The implication is that processes that modulate the pH of seawater on the reef may be more, or as important, as the pH of the open ocean source water to the reef, in determining the acidification of coral reef ecosystems through the 21st century. If future work supports our findings, anthropogenic climate changes that influence coral, coral reef and open ocean productivity will have a major impact on the persistence of coral reefs. 13 peer-reviewed papers, one publically-available software program (coralCT), post-doctoral research, 4 PhD theses, one senior thesis and a PBS NOVA documentary on ocean acidification were supported by this award. Last Modified: 08/22/2016 Submitted by: Anne Cohen