Successive bleaching events cause mass coral mortality in Guam, Micronesia

The reefs of Guam, a high island in the Western Pacific, were impacted by an unprecedented succession of extreme environmental events beginning in 2013. Elevated SSTs induced severe island-wide bleaching in 2013, 2014, 2016, and 2017. Additionally, a major ENSO event triggered extreme low tides beginning in 2014 and extending through 2015, causing additional coral mortality from subaerial exposure on shallow reef flat platforms. Here, we present the results of preliminary analyses of environmental and biological data collected during each of these events. Accumulated heat stress in 2013 was the highest since satellite measurements began, but this record was exceeded in 2017. Overall, live coral cover declined by 37% at shallow reef flat sites along the western coast, and by 34% at shallow seaward slope sites around the island. Staghorn Acropora communities lost an estimated 36% live coral cover by 2017. Shallow seaward slope communities along the eastern windward coast were particularly devastated, with an estimated 60% of live coral cover lost between 2013 and 2017. Preliminary evidence suggests that some coral species are at high risk of extirpation from Guam's waters. In light of predictions of the near-future onset of severe annual bleaching, and the possibility that the events of 2013-2017 may signal the early arrival of these conditions, the persistence of Guam's current reef assemblages is in question. Here, we present detailed documentation of ongoing changes to community structure and the status of vulnerable reef taxa, as well as a critical assessment of our response protocol, which evolved annually as bleaching events unfolded. Such documentation and analysis are critical to formulating effective management strategies for the conservation of remaining reef diversity and function.