By Alistair Bates, ABC
Cyanobacteria and disease has begun to grip the remaining coral. Photo: Supplied: David Juszkiewicz via ABC
On Western Australia's Ningaloo Reef, the shift from stark white to sickly brown paints a dire picture.
"It's a really dramatic change from all the colour and the vibrancy that a healthy coral shows," marine scientist Zoe Richards said.
"We've shifted to this dull phase where it's algae dominating the corals."
The most recent survey by researchers from Curtin University shows that more than 60 percent of corals at eight sites, largely in the reef's northern lagoon, have died.
Of the 1600 corals counted in March, only 600 were still alive come late October.
The findings mark a new phase for the World Heritage site, which was gripped by its worst-ever bleaching event earlier this year.
Scientists warn ocean temperatures off WA are already warmer than average, with the summer months set to be "incredibly critical" for the remaining corals to hang on.
Coral bleaching vs mortality
When the ocean get too hot, corals expel the zooxanthellae algae that lives in their tissue.
These microorganisms give corals their bright pastel colours and resistance to disease.
Their absence creates the bleaching effect many Australians have become all too familiar with.
Unlike its east coast cousin, the Great Barrier Reef, the 270-kilometre Ningaloo Reef largely escaped the impact of climate change-induced ocean warming.
But a record heatwave persisting since December 2024 flipped that fact in the extreme.
"The corals that were bleached earlier in the year have unfortunately succumbed to the heatwave and they have died," Dr Richards said.
"Now, they are literally turned to rock with algae growing on top of them, it's very sad."
Left 'speechless'
PhD candidate David Juszkiewicz said it was not until researchers explored the reef at depth that they realised the extent of the decay.
"It's when you scuba-dive and go down, you realise that there's a lot of death underneath," he said.
Juszkiewicz compared the scene to walking through a forest after a bush fire.
"All the understorey vegetation, everything is burnt-out and charred," he said.
"We're speechless. There [are] no words that can explain what we saw."
Some species are thought to be totally extinct from the reef. Photo: David Juszkiewicz via ABC
Dr Richards said the ripple effect of the coral death was yet to be understood.
"We've lost a lot of the diversity on the reef, the complexity," she said.
"The coral is providing habitat for fish, for crustaceans, for shells, for worms … it's quite likely they have all died."
Dr Richards said the death of the corals was "just the beginning".
"This summer and all the summers to come over the next decade are incredibly critical for these reefs to recover," she said.
There was some hope, however, with corals in the nearby Exmouth Gulf showing signs of resilience.
Crucial window ahead
The creep of rising water temperatures was first detected off WA's coast about a year ago.
Principal research scientist at the Department of Biodiversity, Conservation and Attractions Tom Holmes said the summer months were expected to be less severe.
The department's marine science team is in Exmouth conducting surveys of their own.
"At the same time, in November of 2024, we were already seeing significant build-up of heat stress in the Kimberley, north-west atolls, and even down into the northern Pilbara," Holmes said.
"We're looking probably slightly above average coming into [this] summer, but in the range of 0.4 to 0.8 degrees.
"It probably wouldn't move into the bleaching threshold categories … [although] these are still predictions at this point in time and things can change."
Pockets of hot water holding on from last year's event remained a concern, particularly in the state's south.
"A lot of residual heat was actually retained down in the south-west of WA and that really hasn't cooled coming into this new summer," Holmes said.
"So, we're already looking down at the south-west and seeing some significant heating already starting to occur there."
Rallying for action
Exmouth glass-bottom boat tour operator Craig Kitson said the rotting corals had left him feeling "absolutely helpless".
"The best way I can describe it is if you went out in your backyard and you saw your backyard dying and you didn't know why and there's no fix for it," he said.
Last week Kitson joined a delegation of regional Australians invited to Canberra by the Australian Conservation Foundation to lobby for stronger environmental protection reforms.
He said the situation on the Ningaloo Reef threw the federal Coalition's decision to formally drop its net-zero commitment in sharp relief.
"It's going to need some strong leadership to make the right move because we're staring down the barrel of of the pointy end of climate change," he said.
- ABC
