Deep water no haven from coral bleaching

An international study has discovered coral bleaching caused by warming ocean temperatures is even affecting deepwater reefs.

Corals on the Great Barrier Reef experienced a catastrophic die-off following a 2016 heat wave. (AAP)

Not even reefs in the dimly-lit deep waters of the Great Barrier Reef are protected from bleaching events, a study has found.

International marine scientists working with the University of Queensland have uncovered evidence that the 2016 bleaching on the Great Barrier Reef affected coral up to 40 metres below the surface.

UQ Global Change Institute director Professor Ove Hoegh-Guldberg said the results indicated the limitations of deep reefs as a refuge from thermal anomalies caused by global ocean warming.

"We already established that the refuge role of deep reefs is generally restricted by the limited overlap in species with the shallow reef," Professor Hoegh-Guldberg said.

"However, this adds an extra limitation by demonstrating that the deep reefs themselves are also impacted by higher seawater temperatures."

The depth of deep coral reefs on the Great Barrier Reef make them difficult to study.

The team used remotely operated vehicles to deploy sensors down to 100 metres and characterise temperature differences at depth to those from the shallow reef.

A team of divers then conducted surveys during the height of bleaching, across several sites on the northern Great Barrier Reef.

The study indicates almost a quarter of corals at 40 metres were affected by major bleaching and mortality due to increasing temperatures.

"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," Dr Pedro Frade from the Center of Marine Sciences in Portugal said.

Researchers are now studying how the coral recovery process differs between shallow and deep reefs.