Coral community resilience to successive years of bleaching in Kāne‘ohe Bay, Hawai‘i

The Hawaiian Islands are at the northern edge of coral reef distributions, and corals found there are exposed to large seasonal temperature changes. Historically, coral bleaching in the Hawaiian Islands was extremely rare and had only occurred in 1996. However, in the summers of both 2014 and 2015, successive bleaching events occurred in Kāne‘ohe Bay, O‘ahu. Seawater temperatures were above 28 °C for approximately 1 month in 2014 and 3 months in 2015 and peaked above 30 °C in both years. Patterns of bleaching did not vary among the three sites within Kāne‘ohe Bay. Severe bleaching and paling covered 77 and 55% of reefs in 2014 and 2015, respectively. Different species showed a range of susceptibility with 80–100% of Pocillopora spp. bleaching in both years, but less than 50% bleaching of Porites compressa and Montipora capitata in Kāne‘ohe Bay. Less than 1% of the encrusting coral Leptastrea purpurea colonies bleached in both years. Sixty individual colonies of P. compressa and M. capitata and 28 colonies of Pocillopora damicornis were tagged and monitored for rates of bleaching, recovery and mortality throughout the two-year period. Most of the colonies that bleached recovered their symbionts within 3–4 months, though P. compressa visually recovered more rapidly than M. capitata and P. damicornis. Cumulatively, 19% of P. damicornis, 10% of M. capitata and no P. compressa died by May 2016. Partial mortality within a colony did not occur in 2014, but impacted 13% of the colonies in 2015, with P. damicornis and M. capitata having higher rates of partial mortality than P. compressa. Relatively, low susceptibility in the dominant species and low rates of mortality combined with rapid rates of recovery show coral resilience to anomalously high temperatures in Kāne‘ohe Bay, O‘ahu.