Deep Reefs Were Not Spared During Australia's 2016 Bleaching Season - Bleaching As Deep As 131 Feet

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.

EDIT

https://news.mongabay.com/2018/09/deep-reefs-were-not-spared-by-2016-mass-bleaching-event-on-great-barrier-reef/