The mass bleaching event—that wiped out 30 per cent of shallow-water corals on Australia's iconic Great Barrier Reef in 2016—may have also caused damage in deeper waters, scientists say.
Although deep reefs are often considered a refuge from thermal anomalies, the research, published in the journal Nature Communications, highlights limitations to this role and argues that both shallow and deep reefs are under threat of mass bleaching events.
"During the bleaching event, cold-water upwelling initially provided cooler conditions on the deep reef," said Pim Bongaerts, from the California Academy of Sciences in the US.
"However, when this upwelling stopped towards the end of summer, temperatures rose to record-high levels even at depth," said Bongaerts.
The researchers found bleached coral colonies down to depths of 131 feet beneath the ocean's surface.
"It was a shock to see that the impacts extended to these dimly lit reefs, as we were hoping their depth may have provided protection from this devastating event," said Pedro Frade from the Center of Marine Sciences in Portugal.
The Great Barrier Reef is known to harbour extensive areas of deep coral reefs that are notoriously difficult to study.
Using remotely operated vehicles (ROVs), the team deployed sensors to 328 feet beneath the ocean's surface to characterise how temperature conditions at depth differ from those in shallow habitats.
A team of divers then conducted surveys during the height of bleaching across a number of sites on the northern Great Barrier Reef.
They noted that overall, major bleaching and mortality affected almost a quarter of corals at the deep sampling points, while confirming previous reports of impacts on close to half the shallower corals.
"Unfortunately, this research further stresses the vulnerability of the Great Barrier Reef," said Ove Hoegh-Guldberg from The University of Queensland in Australia.
"We already established that the refuge role of deep reefs is generally restricted by the limited overlap in species with the shallow reef. However, this adds an extra limitation by demonstrating that the deep reefs themselves are also impacted by higher water temperatures," said Hoegh-Guldberg.
