Assessing the geo-ecological consequences of the severe 2023 bleaching event on Mexican Caribbean coral reefs

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Climate change generally, but especially the occurrence of major ocean thermal anomaly events, represents a major threat to the persistence of tropical coral reefs. This threat arises from the fact that when corals are exposed to heat stress (>1 degree C above the annual maximum) and the exposure period exceeds about 4 weeks they respond by expelling their symbiotic algae (zooxanthellae), causing them to turn completely white. If the stress event is very high and/or prolonged the corals will die. Future bleaching projections developed over the past few years have predicted the onset of high frequency bleaching events on many reefs through the next 60-80 years depending on future emission scenarios. However, over the past few months many reefs in the Caribbean have been exposed to temperatures that match these future projections in terms of magnitude and longevity - in essence providing far earlier than expected insights into the future warming regimes reefs may be increasingly exposed to.The Mexican Caribbean region has been no exception. Severe warming started in early May 2023 and measured water temperatures have exceeded 33 degrees C (3 degrees C above the normal seasonal maximum). At the time of writing (early Sept '23) the period of coral bleaching-inducing temperature exposure has already been double the maximum recorded in the region since satellite monitoring began in 1985. Widespread coral bleaching is now evident with unclear consequences for the future ecology and the persistence of reef structures in this region. However, the already very low coral cover on many Mexican Caribbean reefs leaves many reefs at a potentially critical tipping point - whereby any extirpation of what were the remaining and assumed more resilient taxa will see a widespread transition of reefs to states of net erosion over large spatial scales. The implications for the future functionality of the reefs are thus severe. We are in a unique situation to assess the impacts of this bleaching event on both the ecological attributes, but also uniquely the carbonate budget states, of the reefs in this region. Carbonate budget state data is an especially useful reef health monitoring metric from which we can quantity the capacity of reefs to sustain reef framework building and vertical reef accretion potential, functions essential to the persistence of biodiversity and reef-derived services such as coastal wave protection. Our past work, undertaken between 2017 and 2019, involved a major programme of detailed reef carbonate budget assessments at sites along the entire Mexican Mesoamerican reef system (from north of Cancun to the Belize border) and in Veracruz. We propose to undertake a full re-assessment of these sites to provide the most comprehensive reef-tract scale data on the impacts of bleaching that will allow us: 1) to quantify the nature and magnitude of change in coral assemblages at each site, including identification of any between-site variations in species susceptibility; 2) to quantify the magnitude of change in rates of reef carbonate production and bioerosion, and to discern the extent to which taxa are still contributing to the production versus erosion balance; and 3) to quantify the impacts on the vertical accretion potential of these reefs.