Even depths of Great Barrier Reef unsafe from climate change

Even the darkest depths of Australia's world-renowned Great Barrier Reef might not be safe from the impacts of climate change, a New Zealand researcher says.

Victoria University's Dr Alice Rogers was among an international team that investigated a mass bleaching event in 2016 that wiped out around a third of shallow water corals at the Queensland natural wonder.

Coral bleaching, named because of the lightened-white appearance the coral takes when it expels algae from its tissue, is driven by rising sea temperatures.

The new study, published in major scientific journal Nature Communications, showed that while deeper reefs suffered less damage, corals tens of metres below the surface still showed significant signs of bleaching.

"We know little about bleaching temperature thresholds for corals living beyond the well-studied shallow reef," said Rogers, from Victoria's School of Biological Sciences.

"We hope that this study and further science in this area will help us learn more about deep reef response to coral bleaching."

The study's lead author, Dr Pedro Frade from Portugal's Centre of Marine Sciences, was alarmed at the new findings.

"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 team sent remotely-operated vehicles and divers down to survey the reef.

Their findings showed that bleaching affected almost a quarter of corals at a depth of 40 metres, compared to around half the corals at shallower depths.

During the bleaching event, cold-water upwelling initially provided cooler conditions on the deep reef.

One of the research team inspects bleached coral next to healthy-looking seafans. Photo / Pim Bongaerts

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

"Unfortunately, this research further stresses the vulnerability of the Great Barrier Reef," said study co-author Professor Ove Hoegh-Guldberg, of the University of Queensland.

"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."

The research team planned to study the process of recovery from these bleaching events, examining how deeper and shallower reefs differ in their recovery.