Deep reefs were not spared by 2016 mass bleaching event on Great Barrier Reef

New research finds that the mass bleaching event that led to the death of 30 percent of shallow-water corals on the Great Barrier Reef in 2016 also had a substantial impact on deep reefs.

Occurring at depths lower than 30 to 40 meters below the surface of the sea, deep coral reefs, also known as mesophotic reefs, were previously thought to be “ecological refuges from mass bleaching” thanks to cold water rising up from deeper in the ocean, according to a study published in the journal Nature Communications this month that was led by Pedro Fadre of Portugal’s University of Algarve.

Empirical assessments of scientists’ assumptions about the relief from higher ocean temperatures that can be offered to coral by deep reefs are lacking, however, Fadre and team note in the study. They used remotely operated vehicles to deploy sensors up to a depth of 328 feet in order to examine how temperature conditions down below are different from those at shallower depths, and a team of divers then conducted surveys at a number of sites across the northern Great Barrier Reef during the height of the bleaching event.

The researchers ultimately determined that deep reefs’ ability to offer “ecological refuge” has some important limitations, and that both shallow and deep reefs are at risk of mass bleaching in the future.

“We found that summer upwelling initially provided thermal relief at upper mesophotic depths (40 m), but then subsided resulting in anomalously warm temperatures even at depth,” the researchers write in the study. The impacts of bleaching on the deep reefs they studied were “severe,” they add, with 40 percent of coral bleached and 6 percent of colonies dead at 40 meters.

Frade said that he and his colleagues were surprised to discover bleached coral colonies even at a depth of 131 feet. “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,” he said in a statement.

The research team confirmed that deep reefs do offer some relief from thermal stress, as coral on the deep reefs still fared much better than coral at shallower depths of 5 to 25 meters, where 60 to 69 percent were bleached and 8 to 12 percent of colonies died. But there are restrictions on the relief deep reefs can provide: “While we confirm that deep reefs can offer refuge from thermal stress, we highlight important caveats in terms of the transient nature of the protection and their limited ability to provide broad ecological refuge,” the researchers write.

Deep reefs appear to not be protected from “thermal anomalies” driven by global climate change when those higher temperatures extend to later in the year, as they did during the 2016 mass bleaching event, the researchers determined.

“During the bleaching event, cold-water upwelling initially provided cooler conditions on the deep reef,” study co-author Dr. Pim Bongaerts, curator of invertebrate zoology at the California Academy of Sciences in the U.S., said in a statement. “However, when this upwelling stopped towards the end of summer, temperatures rose to record-high levels even at depth.”

The research team also found that the impacts to deep reefs were significant enough that they caused shifts in the coral community structure, which could jeopardize mesophotic reefs’ ability to aid in shallow reef recovery. The researchers say they plan to continue studying how the recovery process varies between shallow and deep reefs.

“Unfortunately, this research further stresses the vulnerability of the Great Barrier Reef,” study co-author Dr. Ove Hoegh-Guldberg of Australia’s University of Queensland said in a statement. “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.”

Another study released earlier this year found that the Great Barrier Reef is losing its ability to bounce back from disturbances like coral bleaching.

CITATIONS

• Frade, P. R., Bongaerts, P., Englebert, N., Rogers, A., Gonzalez-Rivero, M., & Hoegh-Guldberg, O. (2018). Deep reefs of the Great Barrier Reef offer limited thermal refuge during mass coral bleaching. Nature communications, 9(1), 3447. doi:10.1038/s41467-018-05741-0

• Ortiz, J. C., Wolff, N. H., Anthony, K. R., Devlin, M., Lewis, S., & Mumby, P. J. (2018). Impaired recovery of the Great Barrier Reef under cumulative stress. Science Advances, 4(7), eaar6127. doi:10.1126/sciadv.aar6127

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