Shading from topographical complexity mitigates bleaching severity of Pocillopora spp. during thermal stress

Thermally induced bleaching susceptibility of coral varies among species, regional environmental conditions, and local spatial patterns on a reef. As bleaching events become more frequent and severe, understanding the factors that increase resiliency against thermal stress is critical. Here, we used mapping products from large-area imagery collected at the remote atoll of Palmyra in the central Pacific during the 2015 marine heatwave to investigate the role of topographically produced shade in modifying bleaching severity. Change in planar area over 2 yr, bleaching severity, and estimated amount of shade provided by neighboring reef structures were measured for the common coral genus Pocillopora . Bleaching severity was correlated with the estimated irradiance, as colonies in more exposed areas (and thus receiving more daily sunlight) experienced more severe bleaching than corals in more shaded areas. Structural complexity and variability in reef topography provide regions of shade where the compound impact of temperature and irradiance is alleviated, resulting in less severe bleaching outcomes for populations of this cosmopolitan coral taxon. While some studies have considered artificially shading reefs during warm water events, we highlight the importance of existing structural complexity in providing shade, ultimately contributing to the mitigation of thermal impacts in the face of a changing climate.