Successive marine heatwaves cause disproportionate coral bleaching during a fast phase transition from El Niño to La Niña.

The frequency and intensity of marine heatwaves that result in coral bleaching events have increased over recent decades and led to catastrophic losses of reef-building corals in many regions. The high-latitude coral assemblages at Lord Howe Island, which is a UNESCO listed site is the world southernmost coral community, were exposed to successive thermal anomalies following a fast phase-transition of the record-breaking 2009 to 2010 warm pool El Niño in the Central Pacific to a strong La Niña event in late 2010. The coral community experienced severe and unprecedented consecutive bleaching in both 2010 and 2011. Coral health surveys completed between March 2010 and September 2012 quantified the response and recovery of approximately 43,700 coral colonies to these successive marine heatwaves. In March 2010, coral bleaching ranged from severe, with 99% of colonies bleached at some shallow lagoon sites, to mild at deeper reef slope sites, with only 17% of individuals affected. Significant immediate mortality from thermal stress was evident during the peak of the bleaching event. Overall, species in the genera Pocillopora, Stylophora, Seriatopora and Porites were the most affected, while minimal bleaching and mortality was recorded among members of other coral families (e.g. Acroporidae, Dendrophyllidae & Merulinidae). Surviving corals underwent a subsequent, but much less intense, thermal anomaly in 2011 that led to a disproportionate bleaching response among susceptible taxa. While this observation indicates that the capacity of thermally susceptible high-latitude corals to acclimatize to future ocean warming may be limited, particularly if bleaching events occur annually, our long-term survey data shows that coral cover at most sites recovered to pre-bleaching levels within three years in the absence of further thermal anomalies.