Mass bleaching could impact deeper parts of the Great Barrier Reef if rising temperatures are not curbed, scientists say.

The reef is experiencing its fifth mass bleaching event in eight years, due to heat stress.

University of Queensland and University of Exeter scientists examined corals at a depth of 30-50 metres, known as mesophotic reefs that are insulated from the impacts of surface heatwaves.

Professor Peter Mumby from the University of Queensland’s centre for marine science said although these corals are somewhat protected from climate change, there is a limit to their resilience.

“We found separation between warm buoyant surface water and cooler deeper water can insulate reefs from surface heatwaves, but this protection will be lost if global warming exceeds three degrees Celsius above pre-industrial levels,” he said

“Coral bleaching is a dramatic sign of the impact humans are having on the planet and it is increasingly observed at greater depths.”

Researchers estimated temperatures at the bottom of the Great Barrier Reef, could increase by up to 1.7C by 2050-2060.

University of Queensland’s Jennifer McWhorter said warming of three degrees on the surface could push reef temperatures past 30C.

“Some shallow-water coral species are not found in deeper areas – so mesophotic reefs can’t provide refuges for them as shallow reefs are degraded,” she said.

The research is published in the journal ‘Proceeding of the National Academy of Sciences’.

The Barrier Marine Park Authority on Saturday released a reef health update, showing 75 per cent of corals had suffered bleaching.

Half the reef had high or very high levels of coral bleaching.

The update came amid efforts by the federal government to keep the reef off the list of World Heritage Sites in danger.

In-danger listings are not supposed to be a punishment, rather an instrument to encourage a redoubling of conservation efforts.

Keira Jenkins
(Australian Associated Press)