Too hot to handle? The impact of the 2023 marine heatwave on Florida Keys coral

Abstract The marine heatwave in the summer of 2023 was the most severe on record for Florida’s Coral Reef, with unprecedented water temperatures and cumulative thermal stress precipitating near 100% coral bleaching levels. An existing SCTLD coral fate-tracking program assessed over 4200 coral colonies across five offshore and four inshore reef sites approximately every two months, allowing for analyses of bleaching-related mortality and diseases during and after the marine heatwave. Across the vast majority of assessed corals, including multiple sites and species, there was no partial or full mortality as a result of the 2023 bleaching event. The two sites that did experience substantial bleaching-related mortality were those experiencing the highest levels of cumulative thermal stress. However, the substantial acute mortality at one of them occurred at relatively low levels of cumulative stress, suggesting death was the result of exceeding thermal maxima. At the two sites with notable mortality, 43% and 24% of all monitored corals died, but mortality varied among species. Brain corals fared worse than boulder corals, with Pseudodiploria strigosa the most heavily impacted species. The health status of corals before the bleaching event had little impact on whether they experienced disease or bleaching-related mortality during the event. At three sites, we observed unusual lesions on Orbicella faveolata colonies shortly after color returned to the corals; the lesions were only observed for a few months but on some colonies led to substantial tissue loss. Though not part of the coral monitoring program, we also observed substantial losses and local extinctions of Acroporid corals at most sites, as well as probable local extinctions of octocorals at three of the four inshore reefs. Though most reef-building corals came through the 2023 event with no mortality, continually rising temperatures are likely to make these temperature regimes more common and widespread. We encourage future research on what the unusual O. faveolata lesions are, and why the brain and boulder corals fared differently at highly-impacted sites. Our results also provide perspective on how restoration strategies, particularly those focused on species likely to die under current and future climate regimes, should consider shifting focus to species likely to survive. Finally, these results highlight the importance of this type of monitoring, with a focus on fate-tracking individuals through disturbance events, including a large number of individuals of multiple species across a geographic range and multiple habitats.