Hot spots in hot waters: spatial patterns of bleaching-related mortality within individual reefs

Coral mortality resulting from hyperthermal bleaching events has and continues to be a major source of reef decline. Susceptibility to bleaching and associated coral death depends on abiotic as well as biotic variables and as such is known to vary widely both among reefs and within coral colonies. However, susceptibility to bleaching within individual reefs is poorly documented. Spatial variation in susceptibility could indicate abiotic variability within small spatial areas. It could also inform management actions such as likelihood of restoration outplant survival and where to focus bleaching mitigation strategies. We analyzed spatial patterns of bleaching-related mortality at two highly impacted Florida Keys patch reefs during the 2023 marine heatwave. We found significant spatial patterns and hot spots of mortality of Colpophyllia natans colonies at Cheeca Rocks and Pseudodiploria clivosa colonies at Newfound Harbor. We also found strong spatial patterning of black band disease within the same colonies at Cheeca Rocks that overlaid significantly on the bleaching-related mortality hot spots. Though fine-scale abiotic data from the 2023 event are absent, we deployed temperature logger arrays at Newfound Harbor in 2024 and 2025 and found large variation in temperature metrics throughout the site. Maximum degree heating weeks in 2025 ranged from 11.4 – 13.4, and the number of hours over 33.5 °C ranged from 19 to 50 depending on location within the site. These temperature metrics did not correlate well with whether individual colonies exhibited bleaching-related mortality in 2023. However, they were highly correlative with neighborhood means and hot/cold spot locations. This indicates that temperature may have driven neighborhood patterns of mortality, but also that individual colonies within those neighborhoods had varying susceptibilities. We suggest that reefs are likely to have much greater variability in temperature metrics than previously expected, and that this variability may influence the likelihood of bleaching-related mortality. Fate-tracking of individual corals through these events, including species-specific comparisons, can identify these spatial variations and provide valuable information for reef management during bleaching events.