Temperature patterns and mechanisms influencing coral bleaching during the 2016 El Niño

Under extreme heat stress, coralsexpel their symbiotic algae and colour (that is, ‘ bleaching’), which often leads to widespread mortality. Predicting the large-scale environmental conditions that reinforce or mitigate coral bleachingremains unresolved and limits strategic conservation actions1,2. Here we assessed coral bleachingat 226 sites and 26 environmental variables that represent different mechanisms of stress responses from East Africa to Fiji through a coordinated effort to evaluate the coralresponse to the 2014–2016 El Nino/Southern Oscillationthermal anomaly. We applied common time-series methods to study the temporal patterning of acute thermal stress and evaluated the effectiveness of conventional and new sea surface temperature metrics and mechanisms in predicting bleachingseverity. The best models indicated the importance of peak hot temperatures, the duration of cool temperatures and temperature bimodality, which explained ~50% of the variance, compared to the common degree-heating week temperature index that explained only 9%. Our findings suggest that the threshold concept as a mechanism to explain bleachingalone was not as powerful as the multidimensional interactions of stresses, which include the duration and temporal patterning of hot and cold temperature extremesrelative to average local conditions. Improved predictions of coral bleachingare critical. In a coordinated global survey effort during the 2016 El Nino, time-series patterns of peak hot temperatures, cool period durations and temperature bimodalitywere found to be better predictors of coral bleachingthan common threshold metrics.