Anomalous temperatures and extreme tides: Guam staghorn Acropora succumb to a double threat

Anomalously warm sea surface temperature events are increasing in frequency, generating global concern regarding the adaptive and acclimatizing capacities of corals. Staghorn Acropora corals, important ecologically as habitat structurers, are particularly vulnerable to temperature- related bleaching. Here, we report a catastrophic mass mortality event that affected shallow staghorn communities in Guam, Micronesia. Mortality began in conjunction with a mass bleaching event in late 2013, initiated by anomalous warm sea surface temperatures and doldrum winds over a 4 mo period. A second warming event followed less than 8 mo later, concurrent with a period of extreme low tides resulting in repeated periods of subaerial exposure of shallow corals. This combination of stressors acted synergistically to trigger an extended mass mortality event. In 2015, we conducted rapid assessment surveys of 7 species in 21 previously mapped populations to determine mortality extent and pattern. Mortality from these combined environmental stressors resulted in a 53 +/- 10% reduction in Guam's staghorn population, covering an estimated 17.5 ha of coral communities. Greater water circulation appeared to be associated with higher survival during both warm temperature periods and extreme low tides; populations in slightly deeper water, closer to well-flushed reef margins, showed lower mortality. A better understanding of the environmental drivers of the mortality patterns we observed is currently being applied to developing strategies to restore and manage remaining populations.