Tropical corals live at or near their upper thermal limits and are sensitive to periods of elevated sea surface temperatures. As our global climate rapidly warms, the frequency, magnitude, and duration of coral bleaching events is increasing, resulting in widespread losses in coral cover and increased mortality. Yet, corals native to variable thermal reef environments often resist bleaching temperatures, and these habitats have also promoted increased thermal tolerance. On Ofu Island, American Samoa, branching coral species from a highly variable (HV) pool have higher bleaching resistance than corals from a nearby moderately variable (MV) pool. In addition, MV corals were able to increase heat tolerance when moved into the HV pool, providing promising evidence for the persistence of some reefs under projected climate change. In this study, we investigated the breadth of thermal tolerance in massive coral species in the backreef of Ofu Island. We transplanted populations of two massive corals, Porites lobata and Goniastrea retiformis, from three contrasting backreefs separated by months, transplanted and native coral samples were weighed and then exposed to a controlled acute thermal stress assay. Physiological bleaching responses – chlorophyll and photosynthetic efficiency – were quantified to elucidate heat stress resistance. For both species, coral transplants in the HV pool had reduced growth, decreased photosynthetic efficiency, and greater chlorophyll loss following acute heat stress. Variation in growth and thermal tolerance was instead driven by native backreef, not acclimatization or genomic differences. More importantly, the thermal regime of the HV pool surpassed regional records and the magnitude of variability increased, potentially exceeding local stress thresholds of massive coral species. This study strongly contrasts previous research, and could be the first demonstration of a shifting baseline from increased to decreased resilience for corals residing in high-frequency variable environments.https://digitalcommons.odu.edu/sciences_achievement/1012/thumbnail.jp
