Australia’s two world heritage-listed reefs – Ningaloo on the west coast and the Great Barrier Reef on the east – have been hit simultaneously by coral bleaching that reef experts have called “heartbreaking” and “a profoundly distressing moment”.
Teams of scientists on both coasts have been monitoring and tracking the heat stress and bleaching extending across thousands of kilometres of marine habitat, which is likely to have been driven by global heating.
On the Great Barrier Reef, bleaching is being detected from around Townsville to the tip of Cape York, a distance of about 1,000km.
On Western Australia’s famous Ningaloo reef, waters have accumulated the highest amount of heat stress on record during an extended marine heatwave that has hit coral reefs all along the state’s vast coastline.
Paul Gamblin, the chief executive of the Australian Marine Conservation Society, said history would “record this profoundly distressing moment” when two world famous reefs both suffered widespread damage at the same time.
Dr Zoe Richards, an associate professor and coral scientist at Curtin University, spent 10 days monitoring the health of Ningaloo reefs and the neighbouring Exmouth Gulf earlier this month.
She said in shallower areas known for their clear waters, which are popular with tourists, she had seen up to 90% of corals bleached and evidence of corals dying. Even slow-growing corals that were hundreds of years old were bleaching, she said.
Ningaloo last experienced widespread bleaching only three years ago.
The WA government, which is coordinating monitoring across reefs there, said bleaching had also been reported at Kimberley, Ashmore Reef, Rowley Shoals, Barrow Island, Dampier Archipelago, inshore Pilbara and Exmouth Gulf.
Richards said: “This isn’t isolated to Ningaloo – this is happening across the entire north-west shelf. There has never been this scale of impacts in WA. I am not aware of this ever happening before. Climate change has definitely caught up with the reefs in WA.”
Corals lose the algae that give them their colour and most of their nutrients if ocean waters get too warm. If bleaching is not severe, corals can recover, but studies show they are less able to reproduce and are more susceptible to disease.
Coral reef experts use a metric known as degree heating weeks (DHW) to show how much heat corals have accumulated. Generally, corals begin to bleach at about 4DHW, and 8DHW can kill heat-sensitive corals.
Dr Jessica Benthuysen, an oceanographer at the Australian Institute of Marine Science (Aims), first saw signs of heat accumulating in WA last August. By the end of December, she said, some areas had sea surface temperatures 4C hotter than normal.
Benthuysen said levels up to 16DHWs had been detected on the Ningaloo coast, which were the highest on record.
The US government’s Coral Reef Watch says DHWs between 12 and 16 are enough to cause coral death across multiple species.
The federal government’s Great Barrier Reef Marine Park Authority has coordinated monitoring flights over northern reefs, finding low to high levels of bleaching on most reefs. Underwater checks found bleaching at 24 of 30 reefs surveyed.
Bleaching was worse farther north – there is no concern for reefs in the park’s southern section.
Last summer was the worst bleaching event on record for the reef and the fifth major outbreak in eight years, hitting all across the marine park.
Dr Neal Cantin, a coral reef biologist at Aims who was on the monitoring flights, said bleaching was generally worse closer to shore but there was “high to medium” bleaching on reefs from Cairns to the far north. He said in the far north, heat stress was between six and 13DHWs, which was “capable of causing mortality”.
Dr Roger Beeden, the chief scientist at the authority, said detailed analysis of the data from the flights was still being analysed, but he said the lack of recovery time for corals between major events was worrying.
“It’s the frequency as well as the severity that makes us most concerned,” he said.
Dr Emily Howells, a coral scientist from Southern Cross University who has been at the Australian Museum’s research station on Lizard island since February, said this was now the sixth summer in a row that bleaching had been seen there.
The island, in the north of the reef, was badly hit by bleaching last summer and scientists at Aims who visited in subsequent months said the area had lost one-third of its live corals due to the heat.
Howells said there was less coral mortality this year, “but that’s because a lot of the sensitive corals died last summer”.
“There just isn’t enough opportunity for these coral communities to bounce back. It’s heartbreaking,” she said.
“We’re making it more and more challenging for the corals. The solution is having stronger action on climate change. The longer we wait, the worse it will get.”
Northern parts of the Great Barrier Reef have also been heavily affected by flooding from torrential rains. James Cook University’s TropWATER group has recorded flood waters carrying sediments and nutrients in a plume across 700km of the coast and extending as far as 100km offshore.
Jane Waterhouse, a reef water quality expert at TropWATER, said major flood events appeared to be happening more often and flood plumes were reaching farther offshore.
“River discharge carries pollutants, sediments and nutrients,” she said. “You get muddy water that cuts the light that seagrass and corals need to grow, and that nutrient also allows algae to grow.”
Gamblin said the widespread damage from underwater heatwaves and cyclones to both reefs was “what our world-renowned scientists have been warning us about for decades”.
He said fossil fuel companies were “doubling down” to get more mega projects running, pointing to areas around Scott Reef in WA being targeted for expansion by Woodside.
He said: “More mega polluting projects up at places like Scott Reef will make a tragic situation worse. What will our children say to us?”
