Abstract
World-wide coral reefs are in a state of decline as a result of many local and regional factors. Recent global mass mortalities of reef corals due to record warm sea temperatures have led researchers to consider global warming as one of the most significant threats to the persistence of coral reef ecosystems over the next 100 years. It is well established that elevated sea surface temperatures (SSTs) cause widespread coral bleaching, yet confusion lingers as to what facet of extreme temperatures is most important. Utilizing long-term in situ datasets, nine thermal stress indices were calculated and their effectiveness at segregating bleaching years a posteriori for multiple reefs on the Florida Reef Tract was tested. Simple bleaching thresholds based on deviations above the climatological maximum monthly SST were just as effective at identifying bleaching years as complex thermal stress indices. Near real-time bleaching alerts issued by NOAA's Integrated Coral Observing Network (ICON) are now based upon a running 30-day average SST, such that alerts are only issued when the running 30-day average SST exceeds the estimated bleaching threshold for a particular site. In addition to three widespread, mass-coral bleaching events, the Florida Reef Tract was impacted by three tropical storms and 12 hurricanes from 1997-2005. Sea surface cooling associated with the high frequency of hurricanes that impacted Florida in 2005 likely acted to ameliorate the severity and duration of bleaching. Nonetheless, hurricane-associated cooling is not expected to nullify the proposed effects of climate change on coral reefs. The role of thermal stress in coral bleaching has been extensively studied for eastern tropical Pacific (ETP) coral reefs. However, the ETP presents a unique opportunity as this region has sub-optimal conditions for coral reef development because of upwelling of carbon dioxide-enriched deep water along the shallow thermocline. This upwelling results in a depressed aragonite saturation state which is likely an additional factor in the poor reef development throughout the ETP. The highest aragonite saturation documented in this study occur in the Gulf of Chiriquí, which corresponds with the greatest reef development of the entire ETP. Seasonal upwelling had a significant effect on the carbonate chemistry of surface waters in Pacific Panamá. This regionally-depressed aragonite saturation state of the ETP appears to result in corals with a less dense skeleton. Density values of poritid corals from the Galápagos, where aragonite saturation was the lowest documented in this study, were significantly less dense relative to those from Panamá and the Great Barrier Reef. The density of non-living pocilloporid framework components were no different across the ETP aragonite saturation gradients. This could be a result of the activity of boring sponges removing the primary carbonate material within the dead coral skeleton, thus lowering density, albeit physical-chemical dissolution cannot be ruled out. These studies provide real-world examples of the ramifications of global climate change upon coral reef ecosystems.