High-resolution in situ thermal metrics coupled with acute heat stress experiments reveal differential coral bleaching susceptibility

The relationship between thermal stress and coral bleaching has been a topic of study for decades, yet there is still a mismatch between remotely sensed bleaching predictors and population-specific bleaching responses. Recent studies have linked greater amounts of thermal variability to coral bleaching resistance over small spatial scales, yet in some sites, this variability appears to push corals beyond their thermal limits. Here, we performed a 12-month reciprocal transplant and acute heat stress experiments in populations of Porites lobata from an American Samoan backreef: the Highly Variable (HV), the Moderately Variable, and the Less Variable pools of Ofu to investigate how natural and experimental stress across different thermal environments related to coral bleaching responses during the 2016-2017 thermal anomaly. We also compared various sea surface satellite versus in situ temperature data to determine which metrics were most aligned with observed bleaching responses. We found similar to 0.6 degrees C higher maximum monthly means for in situ versus remotely sensed data, which resulted in fewer degree heating weeks. Although greater thermal variability and heat loading did not have a strong relationship with the onset of natural bleaching responses, acute heat stress revealed differences in sublethal stress responses that aligned with in situ thermal metrics. We suggest greater thermal spikes and heat loading in the HV pool, detected only in the high-resolution in situ data, best explain the decreased thermal performance seen in HV corals; demonstrating the utility of in situ data (both environmental and experimental) for understanding bleaching responses at population-specific spatial scales (< 5 km).

Automated summary

The relationship between thermal stress and coral bleaching has been studied for decades, but there is still a mismatch between remotely sensed bleaching predictors and population-specific bleaching responses. This study investigated the relationship between natural and experimental stress and coral bleaching responses in different thermal environments. The results showed that while greater thermal variability and heat loading did not have a strong relationship with the onset of natural bleaching responses, they were correlated with sublethal stress responses. The study highlights the importance of in situ data for understanding population-specific bleaching responses.