A decline in bleaching suggests that depth can provide a refuge from global warming in most coral taxa

Coral reefs are under increasing threat from increasing warm temperature stress. Coral bleaching is caused by a combination of heat and light anomalies and therefore fewer corals should bleach in areas where either heat or light anomalies are ameliorated, such as in turbid waters or at depth. Here, we explore the overall response of the coral assemblage and of 16 individual taxa to a thermal anomaly along a depth gradient during the 2016 mass bleaching event at sites on the outer shelf of the Northern Great Barrier Reef. Across all taxa, there was a curvilinear decline in the percentage of colonies bleached with depth that was consistent among sites and reflected the attenuation of light in the ocean. The percentage of colonies bleached was also higher on reefs with higher levels of temperature stress. In 10 taxa, including the abundant and ecologically significant Acropora, Pocillopora and Porites, the percentage of colonies bleached declined with depth. In 4 taxa, the percentage of colonies bleached peaked at intermediate depth. In 2 taxa, there was no effect of depth because bleaching was uniformly low. These data suggest that deeper areas of reef can provide a refuge from mass bleaching for many colonies of most taxa.