Selective deep water coral bleaching occurs through depth isolation

Climate change is degrading coral reefs around the world. Mass coral bleaching events have become more frequent in recent decades, leading to dramatic declines in coral cover. Mesophotic coral ecosystems (30–150 m depth) comprise an estimated 50–80 % of global coral reef area. The potential for these to act as refuges from climate change is unresolved. Here, we report three mesophotic-specific coral bleaching events in the northern Red Sea over the course of eight years. Over the last decade, faster temperature increases at mesophotic depths resulted in ~50 % decline in coral populations, while the adjacent shallow coral reefs remained intact. Further, community structure shifted from hard coral dominated to turf algae dominated throughout these recurrent bleaching events. Our results do not falsify the notion of the northern Red Sea as a thermal refuge for shallow coral reefs, but question the capacity of mesophotic ecosystems to act as a universal tropical refuge.