Coral bleaching on the Great Barrier Reef not limited to shallow depths

Researcher, Norbert Englebert, running bleaching survey transects, Great Barrier Reef © Pim Bongaerts

A team of international marine scientists working with the ARC Centre of Excellence for Coral Reef Studies at The University of Queensland (Coral CoE) has found evidence to suggest the 2016 coral bleaching on the Great Barrier Reef also affected deep reefs.

Although deep reefs are often considered a refuge from thermal anomalies caused by global ocean warming, the new research highlights limitations to this role and argues that both shallow and deep reefs are under threat of mass bleaching events.

Published in the journal Nature Communications, the new study focuses on the 2016 mass bleaching event that caused the death of 30 per cent of shallow-water corals on the Great Barrier Reef.

The study, details how the impacts of this bleaching did lessen over depth, but were still substantial even on the deep reef.

Lead author Dr Pedro Frade from the Center of Marine Sciences, in Portugal, said the scientists were astounded to find bleached coral colonies all the way down to 40 metres.

"It was a shock to see that the impacts extended to these dimly-lit reefs, as we were hoping that their depth may have provided protection from this devastating event," he said.

The Great Barrier Reef is known to harbor extensive areas of deep coral reefs, however given their depth these reefs are notoriously difficult to study. Using remotely operated vehicles, the team deployed sensors down to 100 metres to characterise how temperature conditions at depth differ from those in the shallow reef.

UQ Global Change Institute Director and Coral CoE's Deputy Director, Professor Ove Hoegh-Guldberg said the study had emphasised the unfortunate vulnerability of the Great Barrier Reef.

"We already established that the refuge role of deep reefs is generally restricted by the limited overlap in species with the shallow reef," he said.

"However, this adds an extra limitation by demonstrating that the deep reefs themselves are also impacted by higher seawater temperatures."

Co-author and Coral CoE alumnus, Dr Pim Bongaerts from the California Academy of Sciences said that during the bleaching event, upwelling cold-water initially provided cooler conditions on the deep reef.

"However, when this upwelling stopped towards the end of summer, temperatures rose to record-high levels even at depth," he said.

A team of divers then conducted surveys during the height of bleaching, across several sites on the northern Great Barrier Reef.

They noted that overall, major bleaching and mortality affected almost a quarter of corals at 40 metres, while confirming previous reports of nearly half the corals being severely affected at the shallower depths.

The researchers have now gone on to study how the process of recovery varies between shallow and deep reefs.