Reduced thermal tolerance of massive coral species in a highly variable environment

Coral bleaching events are increasing in frequency and severity, resulting in widespread losses in coral cover. However, branching corals native to highly variable (HV) thermal environments can have higher bleaching resistance than corals from more moderate habitats. Here, we investigated the response of two massive corals, Porites lobata and Goniastrea retiformis, from a moderately variable (MV) and a low variability (LV) pool transplanted into a HV pool on Ofu Island in American Samoa. Paired transplant and native ramets were exposed to an acute thermal stress after 6 and 12 months of exposure to the HV pool to evaluate changes in thermal tolerance limits. For both species, photosynthetic efficiency and chlorophyll loss following acute heat stress did not differ between ramets transplanted into the HV pool and respective native pool. Moreover, HV native P. lobata exhibited the greatest bleaching susceptibility compared to MV and LV natives and there was no effect of acute heat stress on MV P. lobata. There was also a thermal anomaly during the study, where Ofu's backreef thermal regime surpassed historical records-2015 had 8 degree heating weeks (DHW) and 2016 had up to 5 DHW (in comparison to less than or equal to 3 over the last 10 years)-which may have exceeded the upper thermal limits of HV native P. lobata. These results strongly contrast with other research on coral tolerance in variable environments, potentially underscoring species-specific mechanisms and regional thermal anomalies that may be equally important in shaping coral responses to extreme temperatures.