Elucidating Factors Underlying Variation in Bleaching Response on Reefs in Southeast Florida During the 2023 Marine Heatwave

Record-breaking temperatures during a marine heatwave in the summer of 2023 brought unprecedented heat stress to reefs throughout Florida, with reefs in the Florida Keys generally experiencing severe bleaching (loss of pigmentation due to expulsion of algal symbionts) and mortality, particularly of acroporid corals. Reefs off Miami, however, experienced only mild bleaching, with a distinct break in bleaching severity around Fowey Rocks, in central Miami-Dade County. Using standardized coral bleaching surveys from the Disturbance Response Monitoring (DRM) program, temperature, depth, water quality, historical bleaching, and algal symbiont community data were analyzed through multivariate approaches to identify the drivers of differential bleaching response between northern and southern reefs. While no single factor fully explained the observed patterns, we found that thermal stress, light availability, and nutrient differences all influenced coral responses. We hypothesize that Miami’s urban-influenced reefs, which experience more variable environmental conditions, may have acclimatized or adapted to chronic stress, enhancing their resilience to extreme heatwaves. In contrast, reefs to the south of urban Miami-Dade, which faced greater environmental extremes in 2023 than in previous years, may have been less prepared for the unprecedented conditions. This study highlights the importance of understanding the influence of multiple interacting factors on coral bleaching susceptibility. These findings can inform local reef management strategies and underscore the need for further research into the role of urban reefs in climate resilience and restoration efforts.