coral crisis continues to unfold, once shocking news has become commonplace: Case in point, Australian scientists now report that coral cover on the Great Barrier Reef has fallen sharply in all three sections of the reef for the first time in 35 years of continuous monitoring.
Their recently completed annual survey follows the back-to-back bleaching events of 2016–17 that killed off half of the Great Barrier Reef’s corals. After surveying 50 reefs in the Great Barrier Reef system between September of 2017 and May of 2018, researchers at the government-funded Australian Institute of Marine Science found that mean coral cover had fallen from 22 percent to 14 percent in the central section of the reef over the past year, and from 33 percent to 25 percent in the southern section. Reefs in the northern section were not surveyed, but reports from aerial surveys showed that mean coral cover had plunged to just 10 percent in 2017, according to the AIMS report.
bleaching events occur with increasing frequency as greenhouse gas emissions accelerate climate change. (When ocean temperatures exceed corals’ tolerance, the symbiotic algae that supply their food and color in exchange for shelter turn toxic and are expelled. Deprived of nutrition, corals turn white and can die unless waters cool.)
Terry Hughes and his team indicates that “recovery will be slow and painful and will likely be mostly local and it won’t go back to the same system,” as he told attendees at the annual meeting of the Australian Coral Reef Society in May.
“The barrier reef has just gone through one hell of a natural selection event,” he said. “To me, the bleaching is a game changer. It’s a new system.”
Hughes and his colleagues laid “recruitment panels” along the length of the Great Barrier Reef to collect coral larvae. They then retrieved the panels in January of 2018 to determine how many larvae had successfully spawned and settled on reefs where they would grow into new coral colonies, a process known as recruitment.
“The amount of recruitment this last season is a tiny, tiny percentage of the normal level of recruitment, which is directly related to the loss of adult brood stock,” Hughes, the director of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University, told the gathering of coral scientists in Exmouth, Australia.
Hugh Sweatman, a senior research scientist at AIMS and the leader of the organization’s Long-Term Monitoring Program, said in an email that healthy coral cover could be considered 75 percent or greater, though that is “quite rare in my 40 years’ experience.”
He noted that, over thousands of years, coral ecosystems have faced ongoing disturbance from cyclones, crown-of-thorns starfish infestations, and sediment runoff that blocks sunlight. “Thus a reef can be ‘healthy’ in terms of retaining ecosystem functions and capacity to recover, but if you survey it soon after a disturbance, the coral cover will be low,” Sweatman said. “The more revealing measure of reef health and ecosystem function is the rate at which coral cover returns.”
With some 3,300 individual reefs spread over 133,000 square miles of ocean along 1,400 miles of Australia’s northeast coast, the Great Barrier Reef, as Sweatman notes, “is a big place, and few disturbances affect the entire area.”
Yet the back-to-back bleaching events of 2016–17 struck two-thirds of the Great Barrier Reef. The southern third was spared extensive bleaching only by the propitious appearance of a cyclone that cooled waters during the first bleaching event. Storms, meanwhile, are increasingly intense and causing more damage to coral reefs. Then there are the outbreaks of crown-of-thorns starfish that eat corals.
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