Mesophotic coral bleaching associated with changes in thermocline depth

Abstract

As global temperatures continue to rise, shallow coral reef bleaching has become more intense and widespread. Mesophotic coral ecosystems reside in deeper (30–150 m), cooler water and were thought to offer a refuge to shallow-water reefs. Studies now show that mesophotic coral ecosystems instead have limited connectivity with shallow corals but host diverse endemic communities. Given their extensive distribution and high biodiversity, understanding their susceptibility to warming oceans is imperative. In this multidisciplinary study of an atoll in the Chagos Archipelago in the central Indian Ocean, we show evidence of coral bleaching at 90 m, despite the absence of shallow-water bleaching. We also show that the bleaching was associated with sustained thermocline deepening driven by the Indian Ocean Dipole, which might be further enhanced by internal waves whose influence varied at a sub-atoll scale. Our results demonstrate the potential vulnerability of mesophotic coral ecosystems to thermal stress and highlight the need for oceanographic knowledge to predict bleaching susceptibility and heterogeneity.

Rising global temperatures cause widespread bleaching of shallow coral reefs but mesophotic reefs at depths over 30 metres are thought to be sheltered by cooler waters. Here, at sites in the Chagos Archipelago, the authors show bleaching of corals at depths of 90 metres, which might be due to warm surface waters being pushed deeper by the ocean’s response to the Indian Ocean Dipole.