Award: OCE-1031971

Coral reefs support the livelihoods of 500 million people worldwide, provide billions of dollars in coastline protection, and harbor 25% of all marine species. Yet as tropical sea surface temperatures (SSTs) have increased, coral reefs around the world are responding in widespread bleaching events leading to irreversible coral mortality and permanent loss of reef area in many regions. Atmospheric CO2 continues to rise, and the ocean continues to warm. Global climate models (GCMs) project more than 3°C warming by 2099, which if realized, puts the survival of tropical coral reef ecosystems at risk. This project addressed three key questions: (1) GCMÆs project open ocean conditions but what are the spatial and temporal patterns of the predicted warming at the scale of coral reefs, (2) Corals bleach when the water gets too warm but thermal thresholds differ amongst populations and species; some are more thermally tolerant than others. In the absence of comprehensive bleaching survey data, can we develop tools to enable identification of coral population responses to past periods of known thermal stress? (3) When we apply these tools to identify coral community responses to thermal stress, what patterns emerge? Can we relate thermal tolerance and sensitivity to history of exposure and/or to modulation of thermal regime by oceanographic circulation patterns that operate on a local scale? Combining a high resolution Ocean GCM with satellite measurements and a unique decade-long set of in situ coral reef temperature data, we showed that ocean warming around coral reefs in the central equatorial Pacific has, and will continue to be modulated by a strengthening of the Equatorial Undercurrent (EUC) that brings cool, nutrient rich water to the surface at several islands within the Pacific Remote Marine National Monument. The degree of modulation afforded by a strengthening EUC over the next few decades will depend on whether the projections of the GCMÆs are realized or whether historical trends will continue. We show that corals exhibiting stress during a high temperature event record that response in their calcium carbonate skeletons, by depositing a stress band, and by decreasing their investment in skeletal growth. We developed an automated program to quantify these responses in 3-D CAT scan images of skeletal cores removed from corals in the Caribbean, western and central Pacific. By comparing bleaching survey data with stress band data, we show that the proportion of skeletal stress bands in a population reflects the extent of community scale bleaching. Therefore, skeletal stress bands are a proxy for bleaching events and can be used to reconstruct coral reef responses to thermal stress across space and through time. Application of this new proxy shows a systematic increase in levels of thermal stress in coral communities around Puerto Rico, from 25% of the community stressed in 1983 to 90% stressed in 2005. This observation is consistent with the rapid decline in coral cover on these reefs over the corresponding time period. On Palau in the western tropical Pacific, we show that corals previously believed to be thermally tolerant were thermally stressed during the 2010 ENSO event. Coral communities in PalauÆs Rock Islands did not bleach in 1998 while coral communities on the barrier reef were severely affected. However, in 2010, our data show 30% of the Rock Island community bleached, suggesting a decline in thermal tolerance over time. This is consistent with similar changes observed amongst coral communities in the Caribbean. In the central Pacific, we compared coral communities which by virtue of their location, experience different levels of influence of the EUC. Our data suggest that corals on Jarvis Island, where the EUC influence is strongest, exhibit minimal bleaching during high temperature events, whereas corals on Maiana where EUC influence is weaker, exhibit higher levels of bleaching. These results are consi...