Meeting Abstract
Coral bleaching events (i.e., dis-association between coral animals and their dinoflagellate photosymbionts) are increasing in frequency and severity, resulting in widespread losses in coral cover and an urgency to identify resilient populations. Recently, research has found stress-tolerant coral populations that are adapted to highly variable environments and possess greater bleaching resistance than corals from more moderately variable habitats. Using well-studied, environmentally variable backreef lagoons (Ofu Island, American Samoa), we evaluated the thermal tolerance scope of the massive coral Porites lobata following a reciprocal transplant experiment between a Moderately Variable (MV) and Highly Variable (HV) pool, as well as transplanting from a Low Variability (LV) pool into the HV pool. Transplanted and native samples were exposed to a controlled acute thermal stress throughout a two-year transplant period. Corals transplanted into the HV pool had reduced growth, decreased photosynthetic efficiency, and greater chlorophyll loss following acute heat stress compared to native back-transplants in their pool of origin. HV corals grew the most yet exhibited the greatest bleaching susceptibility compared to MV and LV natives. Surprisingly, MV native corals were resilient to acute thermal stress. In contrast to previous studies, there was a thermal anomaly in the region where Ofu’s backreef thermal regime surpassed historical records – 2017 had up to 9.5 Degree Heating Weeks (DHW), 2016 and 2015 had up to 8 and 5 DHW, respectively (in comparison to ≤ 3 over the last 10 yrs). These results indicate the HV environment greatly exceeded historical variability and could be reaching a tipping point from enhancing coral stress tolerance to potentially overwhelming upper tolerance limits.
