Assessment of acclimatization and subsequent survival of corals during repeated natural thermal stress events in Hawai'i

The ability of an organism to respond to a changing climate is an important issue for the future of coastal ecosystems worldwide. Coral bleaching is a stress response to elevated seawater temperatures, which are projected to increase as a result of a warming climate. Coral populations adjust their thermal sensitivity through adaptation or individual colony acclimatization, but identifying acclimatization of individual colonies requires long-term ecological observation in the field. Consecutive bleaching events in Hawai'i in 2014 and 2015 provided an unprecedented natural experiment for comparing bleaching susceptibilities of coral communities as well as assessing acclimatization. Individual colonies were monitored for 15 mo encompassing the periods before, during, and after both bleaching events. Metrics of bleaching, recovery, and mortality were calculated to investigate responses between bleaching events. Initial colony responses varied by species, with only Porites evermanni exhibiting significantly less bleaching in the second year despite a 27 % increase in accumulated thermal stress and low partial mortality overall. In contrast, P. lobata and Pocillopora meandrina experienced similar bleaching responses both years, with P. lobata experiencing significantly less mortality the second year, and P. meandrina having similar mortality both years. This study demonstrates the importance of monitoring individual colonies to fully understand reef bleaching dynamics and points to the influence of species composition when assessing the potential for reef acclimatization. Given that thermal stress is predicted to be a major stressor for coral reefs in the future, acclimatization may serve as a critical mechanism that limits mass mortality, allowing time for populations to adapt.