Coral cover and species responses to heat exposure vary across contemporary Western Atlantic reefs.

Ocean warming threatens the persistence of tropical corals and the biologically diverse ecosystems they sustain. While field-based studies on heat impact have predominantly focused on quantifying coral bleaching, a symptom of thermal stress, less attention has been given to understanding trends in coral mortality, a critical metric for assessing and predicting the long-term effects of rising temperatures. Consequently, the relationships between varying heat exposures and resultant coral cover changes remain poorly quantified. Such trends are challenging to establish in the Tropical and Subtropical Western Atlantic (TWA), because climate change impacts are compounded by local anthropogenic and natural disturbances. Additionally, many coral communities have already been substantially altered, with those remaining dominated by relatively resilient species. This study addresses this issue by quantifying coral cover loss as a function of maximum Degree Heating Weeks (DHW) exposure using observed in-situ coral cover changes across reefs in the TWA. Of the five locations assessed (the Florida Keys, Dry Tortugas, Puerto Rico, the US Virgin Islands, and the East and West Flower Garden Banks), all exhibited significant declines in mean coral cover with increasing DHW exposure. Rates ranged from 0.3% to 2.4% annual loss in relative mean cover per unit DHW, with spatial variability largely reflecting pre-impact system conditions and variations in the life-history traits of geographically distinct coral assemblages (e.g., Puerto Rico vs East and West Flower Garden Banks). Variation in responses to DHW among species was also observed across locations. By establishing site-specific coral loss parameters, this study contributes to our understanding of how future coral cover may evolve under escalating thermal stress in the TWA. It also provides practical guidance for targeted restoration by identifying species that have fared comparatively well across locations, grounding efforts in what is possible under current and future conditions rather than idealised historical baselines.