Coral reefs in the Mahafaly Seascape (SW Madagascar) as potential climate refugia following the 2024 mass bleaching event

The southwesternmost coast of Madagascar, including the Mahafaly seascape, is home to some of the country's most ecologically and culturally important coral reefs, which remain largely understudied. These ecosystems are facing growing pressure from a panel of disturbances, with climate-induced events such as marine heatwaves being among of the causes of coral bleaching. The decline of these reefs can directly threaten the livelihoods of the local communities, who depend heavily on the resources they provide. In this context, understanding how different disturbances impact coral reef condition and recovery is critical for developing effective management strategies and conservation for this vulnerable region. This study investigates coral bleaching dynamics in the Mahafaly seascape during and after the fourth mass coral bleaching event in 2024. Six reef locations within locally managed marine areas were surveyed, comprising a total of 11 stations, with 20 one-square-meter quadrats randomly deployed at each station during each sampling period. Average hard coral densities of 18.3 and 19.3 colonies m^-2 were recorded during and after the event, respectively. Bleaching prevalence peaked during the event, affecting 38.8% of coral colonies, with a marked decrease to 6.0% afterward. However, coral mortality remained relatively low across the entire seascape, with a slight post-bleaching increase in dead colonies (+3.5%). Northern sites experienced the highest thermal stress, with Beheloke and Besambay reefs being the most susceptible to bleaching, whereas Ambola exhibited remarkable resilience. In the south, Ambohibola showed low vulnerability, Lembehitake moderate susceptibility, and Itampolo the highest bleaching prevalence despite lower thermal stress. Taxa-specific patterns were also evident, with thermally sensitive branching genera being most affected, contrasting with the relative tolerance of massive and encrusting corals. The heterogeneous coral bleaching patterns observed across the seascape emphasize the importance of site- and taxa-specific monitoring to prioritize management actions where reef resilience is highest or degradation most severe. Despite bleaching severity, the results suggest that coral assemblages in the seascape have a strong capacity to recover following the event. The Mahafaly seascape reefs could serve as vital refugia in the face of climate change, underscoring the need for ongoing conservation efforts.