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

The frequency and intensity of marine heatwaves resulting in thermal bleaching events have increased over recent decades leading to catastrophic losses of reef-building corals in many regions. Successive annual bleaching events are also becoming more frequent, limiting the capacity of susceptible coral species to recover. Following an unusual fast phase transition of the record-breaking 2009–2010 warm pool El Niño event in the Central Pacific to a strong La Niña event the following year, high-latitude coral reef assemblages around Lord Howe Island were exposed to unprecedented successive thermal anomalies causing severe bleaching. Coral health surveys completed between March 2010 and September 2012 quantified the response and resilience of approximately 42,000 coral colonies from different taxa to the successive bleaching events. Changes in benthic community composition before, during and after the thermal stress events were also assessed. In March 2010, severe coral bleaching ranged between 99% at some shallow lagoon sites to 17% at deeper reef slope sites. Significant coral tissue mortality was evident during March and May 2010, with increased pigmentation and colour returning to surviving colonies by September 2010, indicating recovery of symbiotic function in living coral colonies. Pocillopora, Stylophora, Porites and Montipora species were the most affected taxa, with minimal mortality observed in merulinid and Acropora species. During the second thermal anomaly in 2011, significant bleaching occurred in susceptible coral taxa, demonstrating limited resilience and acclimation capacity of these high-latitude corals to future-ocean warming. Repeated bleaching stress resulted in a shift at some sites from a coral-dominated reef assemblage to one comprising a higher cover of macroalgae and other invertebrate taxa. These findings demonstrate that future-ocean warming and extreme heatwave events are likely to lead to significant shifts in reef assemblages and potential local extinction of some dominant but vulnerable reef-building corals at this world heritage listed site. <b>Citation:</b> Steven J. Dalton, Andrew G. Carroll, Eugenia Sampayo, George Roff, Peter L. Harrison, Kristina Entwistle, Zhi Huang, Anya Salih, Sandra L. Diamond, Successive marine heatwaves cause disproportionate coral bleaching during a fast phase transition from El Niño to La Niña, <i>Science of The Total Environment</b>, Volume 715, 2020, 136951, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2020.136951.