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Coral Cover on Great Barrier Reef Is in Steep Decline

In the wake of catastrophic bleaching events, scientists find that coral cover on the world’s biggest reef system is near record lows, while other research shows recovery will be “slow and painful” as only a tiny number of coral larvae are repopulating devastated reefs.

Written by Todd Woody Published on Read time Approx. 5 minutes
Bleached corals off Heron Island in the Great Barrier Reef.The Ocean Agency / XL Catlin Seaview Survey

As the global 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 2017 and May 2018, researchers at the government-funded Australian Institute of Marine Science (AIMS) 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.

Now scientists are questioning whether those reefs – which also face threats from cyclones, pollution and predatory crown-of-thorns starfish – will be able to bounce back, given that 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.)

“The geographic scale of recent bleaching means that breeding populations of corals have been decimated over large areas, reducing the potential sources of larvae to recolonize reefs over the next years,” the AIMS scientists wrote in their report. “It is unprecedented in the 30+ year time series that all three regions of the [Great Barrier Reef] have declined and that many reefs have now very low coral cover.”

Separately, new research by coral scientist 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.”

GBR bleaching 2016-2017.jpg

Great Barrier Reef bleaching 2016–17. (ARC Centre Of Excellence for Coral Reef Studies)

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 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.

That means there are far fewer corals to recolonize devastated reefs. In the course of their fieldwork, the researchers also surveyed the number of juvenile corals. “The juveniles seemed to have survived better than adults,” he said. “In most places we’ve looked, the juvenile density is half what it used to be.”

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 344,400 square km (133,000 square miles) of ocean along 2,300km (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.

Bleached corals on the Great Barrier Reef. (The Ocean Agency/XL Catlin Seaview Survey)

All of which “means that the intervals between the acute disturbances decreases – less time for recovery to occur,” said Sweatman. “At the same time, coastal development often leads to more sediment and pollutants, which slow the process of recovery (plus there are coral diseases and sub-lethal temperatures which may also slow recovery). So greater loss of coral must be made good in shorter times as recovery rates slow … they all point to lower coral cover.”

Hughes and other coral scientist have repeatedly said the only long-term hope for the world’s coral reefs is a dramatic reduction in greenhouse gas emissions driven by the burning of fossil fuels.

“Personally, I am not totally pessimistic,” Sweatman said. “In the short term, the north and central [Great Barrier Reef] reefs may substantially recover. But in the longer term the outlook is very poor unless there are dramatic changes.”

For Hughes’ part, “I’m scraping the barrel here looking for some sources of optimism,” he told his fellow coral scientists at the conference in May. “The glass is still half full. About 51 or 52 percent of the corals that were alive two years ago are still out there.”

“So my final take-home point is that we need to look after these billion survivors, and I don’t think there’s much we can do in terms of restoring them based on growing corals in aquariums,” he added.

This article has been corrected to reflect that while coral cover is in steep decline in all three sections of the Great Barrier Reef for the first time in 35 years of monitoring, coral cover is not at a record low. 

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