Deep coral reefs on the Great Barrier Reef are shown to have lower levels of bleaching than shallower reefs during the 2016 mass bleaching event, according to a paper in Nature Communications. The study suggests that deep reefs may offer some refuge from thermal stress, but the nature of the protection may be transient and limited in providing broad ecological refuge.

It has been suggested that coral reefs found in deep waters (at depths of 30 to 40 metres and beyond) could act as major ecological refuges from mass coral bleaching. However, assessments have been limited because of the logistical complexity of studying these ecosystems.

Pedro Frade and colleagues studied the amount of bleaching experienced by deep reefs at nine sites within the Great Barrier Reef during the large-scale bleaching event of 2016. Impacts on the deep reefs were severe (40% of coral was bleached and 6% dead at 40 metres) but lower than at shallower depths (60-69% of coral was bleached and 8-12% dead at 5-25 metres). The authors found that upwelling of cold water gave deeper reefs some protection in the early summer, but this was lost by late summer when upwelling stopped.

The study highlights the limitations of deep reefs acting as thermal refuges, and the authors argue that both shallow and deep reefs are under threat from mass bleaching events.