Patterns of bleaching and recovery of Montipora capitata in Kane'oha Bay, Hawai'i, USA

As ocean warming causes more frequent and severe coral bleaching worldwide, it is critical to identify biotic and abiotic factors that promote bleaching resistance and recovery. In October 2014, many colonies of the key reef-building coral Montipora capitata in Kane'ohe Bay, O'ahu, Hawai'i, USA, were severely bleached, while others appeared unaffected. To elucidate the role of symbiotic algae in these contrasting responses and study subsequent patterns of recovery, we tracked abundances (symbiont to host cell ratios) of clade C and D Symbiodinium for 6 mo in 10 bleached and 10 non-bleached colonies at 3 reefs in the northern, central, and southern regions of Kane'ohe Bay (n = 60 colonies) using quantitative PCR. Bleaching resistance was significantly associated with the dominant symbiont clade. All bleached colonies (n = 30) were dominated by clade C symbionts, while many non-bleached colonies (n = 16) were dominated by thermotolerant clade D. However, clade C Symbiodinium dominated 14 other colonies that did not bleach, indicating that an alternate mechanism such as host genetic adaptation may play a role in thermal tolerance of these colonies. Bleached corals recovered their symbionts within 1-3 mo (excepting 1 mortality) and remained C-dominated. However, colonies recovered 3 times faster at the northern reef, which experiences similar temperature but lower irradiance and higher water flow and turnover compared to the southern reef. This work indicates that both biotic (e.g. symbiont and host genotypic) and abiotic (e.g. hydrodynamic) factors influence the natural resistance and recovery of M. capitata, which can inform ecological predictions and conservation strategies for coral reefs under climate change.