New insights into coral bleaching on the Great Barrier Reef
A new study of mass coral bleaching on the Great Barrier Reef suggests the deeper reefs may not be a refuge from temperature anomalies caused by global warming.
Victoria University of Wellington scientist Dr Alice Rogers was part of an international research team whose findings, published in Nature Communications, showed coral bleaching affected both shallow and deep reefs.
“We know little about bleaching temperature thresholds for corals living beyond the well-studied shallow reef,” says Dr Rogers from the University’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 focused on a mass bleaching event in 2016 that caused the death of 30 percent of shallow-water corals on Australia’s Great Barrier Reef. It showed that deeper reefs suffered less damage, but that corals up to 40 metres under the sea still showed significant signs of bleaching.
“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,” says Dr Pedro Frade from the Center of Marine Sciences (CCMAR) in Portugal, lead author on the study.
The research team sent both 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. However, when this upwelling stopped towards the end of summer, temperatures rose to record-high levels even at depth,” says co-author Dr Pim Bongaerts from the California Academy of Sciences.
“Unfortunately, this research further stresses the vulnerability of the Great Barrier Reef,” says Professor Ove Hoegh-Guldberg from 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. However, this adds an extra limitation by demonstrating that the deep reefs themselves are also impacted by higher seawater temperatures.”
The research team now plan to study the process of recovery from these bleaching events, examining how deeper and shallower reefs differ in their recovery.