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Climate change

One third of Great Barrier Reef corals killed in marine heatwave in 2016

Coral bleaching occurs when high water temperatures cause corals to expel the algae, making the coral pale or white

PUBLISHED : Thursday, 19 April, 2018, 1:20pm
UPDATED : Thursday, 19 April, 2018, 10:50pm

High water temperatures off the east Australian coast in 2016 bleached and “cooked” large sections of the Great Barrier Reef, according to a new scientific study released on Thursday.

The study, published in the journal Nature, showed 29 per cent of the almost 4,000 reefs that make up the Great Barrier Reef lost at least two-thirds of their corals over a nine-month period, affecting their ability to support diverse ecosystems.

“When corals bleach from a heatwave, they can either survive and regain their colour slowly as the temperature drops, or they can die,” Terry Hughes, lead study author and director of the Australian Research Council Centre of Excellence for Coral Reef Studies, said in a statement.

Corals, or coral polyps, are soft-bodied organisms that have a symbiotic relationship with algae living in their tissues. Algae provide food and colour to the corals.

Coral bleaching occurs when high water temperatures cause corals to expel the algae, making the coral pale or white.

The study found corals in the northern third of the reef, where water temperatures were most extreme, were the most severely affected, with half of the coral cover in the most shallow areas dying in an eight-month period.

They cooked because the temperatures were so extreme
Terry Hughes, expert

Hughes told the Australian Broadcasting Corporation that heat stress – not starvation caused by bleaching – was the direct cause of coral death.

“They cooked because the temperatures were so extreme,” he said, referring to temperature-sensitive species of corals which began to die almost immediately when water temperatures rose.

Researchers used aerial surveys to measure the extent and severity of bleaching along the 2,300km-long reef, while the geographical pattern of heat exposure was mapped from satellites.

The paper says extreme weather events, caused by man-made climate change, are “rapidly emerging as major contemporary threats to almost all ecosystems,” and suggests that while not all corals will be killed off, the shape and variety will be affected.

“[Corals that survived the bleaching event] are tougher than the ones that died. We need to focus urgently on protecting the glass that’s still half-full, by helping these survivors to recover,” Hughes said in a statement.

Researchers warn that failure to prevent global temperatures from rising above 2 degrees Celsius from pre-industrial levels will radically alter tropical reef ecosystems.

“The Great Barrier Reef is certainly threatened by climate change, but it is not doomed if we deal very quickly with greenhouse gas emissions,” Hughes said.