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There aren’t many places as idyllic as a tropical coral cay on the Great Barrier Reef (GBR). These hard-to-reach places – natural mounds of white coral sand and rubble that often sprout lush green vegetation fertilised by seabirds – are among the planet’s most picturesque places. But the mood is sombre among the researchers who greet me and photographer Craig Parry as we arrive in early May 2024 on the RV Linckia II at The University of Sydney (USyd) research station on One Tree Island (OTI), a 4ha cay off the Queensland coast, about 100km east of Gladstone. Among them is Dr Steph Gardner, a marine microbial ecologist from The University of Sydney who, like so many reef researchers, has been in love with the GBR – its colour, movement and abundance of life – since childhood. 

Steph has been here since late March following up on research she began in 2023, looking at bacterial diversity in the sand around corals. Although it’s not yet clear what role these life forms might play, they could be critical to coral-reef health, just like healthy gut biomes are known to be important to human wellbeing. When Steph first arrived here this season, she couldn’t wait to show her colleagues Raphael Burkart-Radtke and Ana Olmos-Pin the beauty in the famously untouched waters around OTI. As one of only two “orange” research zones closed to the public within the massive 344, marine park that protects much of the GBR, the OTI environment is as close to pristine as you can get. It’s off-limits to tourists, only low-impact research is allowed and there’s virtually no run-off of any sort from operations here. 

Showers are limited to one bucket of fresh water a day per person, collected from rainwater tanks; only reef-friendly sunscreens and soaps are allowed, and toilet facilities are the sustainable long-drop pit variety. Heinrich Breuer, who manages the island with partner Ruby Holmes, even refuses to use potentially polluting antifouling chemicals on the research vessels that service the island. He opts instead to treat them by periodically hauling them out of the water so they can be exposed to natural UV light from the sun, which helps kill any algae growing on the bottom of the boats. 

The near-pristine environment of One Tree Island makes it a perfect outdoor laboratory for studying the effects of climate change, bleaching and eutrophication on reef systems. Located at the south-eastern end of its own 5.5 x 3.5km reef, the tiny island lies in the centre of the GBR’s Capricorn Group, 20km east of Heron Island and 100km from the coal port of Gladstone. In 1965 the Australian Museum established a research station here that’s been managed by The University of Sydney since 1974.

All this has meant that underwater life around OTI in the 21st century has been flourishing unlike any other study location along the GBR, as it has done for millennia, with very little external influence. The reef here survives as it’s meant to, and has been well studied as it does so for more than 50 years. Even the notorious coral-munching sea star species known as crown-of-thorns (COTS), which is endemic to the GBR but can periodically reach plague proportions, seems to have been naturally kept in check at OTI, without any culling or other extraneous measures. USyd PhD student Matt Clements has been studying the COTS population around the island for the past four years, coming here up to four times a year to do so. “The interesting thing about One Tree is that the COTS population here has been stable and low density for 10-plus years,” Matt explains. “And there’s never been a documented [population] outbreak here, ever, which is interesting.” 

Even when Steph heard from other researchers that seawater temperatures last summer had been the highest recorded at OTI in 50 years, she was hopeful the pristine cay might have been able to hold its own against the bleaching crisis playing out elsewhere on the reef. It couldn’t.

On her first dive on the island this year, Steph was met with widespread signs of death and destruction. “I broke down three times that day,” she recalls. “I was physically hurt seeing what it had done to the reef, seeing the corals like that, some already bleached and white, others fluorescing [a sign of stress in coral]. “It just killed me,” she says. “We do this [research] work because we love the reef and it’s usually beautiful here. This just felt so wrong.” 

A woman standing amongst endemic pisonia trees on Lady Elliott Island. Related: The storytellers of the Great Barrier Reef

Widespread devastation

This most recent mass bleaching on the World Heritage- listed GBR, that occurred in 2023–24, is the fifth since 2016. Previous events have hit the northern sections of the reef hard, but sometimes spared southern GBR locations, such as Heron Island, about 20km west of OTI, and Lady Elliot Island, 90km south of OTI, from the worst impacts. This time the impact has been felt right along the GBR’s entire 2300km length. A report released in April by the GBR Marine Park Authority (GBRMPA) and the Australian Institute of Marine Science estimated that up to three-quarters of the 3000 individual reefs in that massive area have been affected to some extent.

Associate Professor Chris Roelfsema, academic director of The University of Queensland’s Heron Island Research Station, has been coming to Heron annually since 1998 and surveying coral cover here using field photographs and satellite imagery since the early 2000s. From 2008 right up until early 2024 he’d actually documented increasing coral cover around the island, which is an 80km ferry ride east of the coal port of Gladstone. Chris had never seen a major bleaching event in the area but knew it was only a matter of time.

“For the last five or six years, every time I left the island I’d wonder what it would look like when I returned,” he says. When higher-than-normal ocean temperatures hit the reef for extended periods from December 2023, he advised Heron researchers to be on high alert, ready to ramp up and expand their usual research projects to gather as much information as possible about the bleaching event that the conditions warned was coming. Still, no-one expected what was to unfold in the waters around Heron last summer.

Heron is not as protected as OTI. But it’s a beautiful island that’s relatively easy to reach and well known for capturing the heart of famed naturalist Sir David Attenborough, who filmed here in 2009.

Dr Caitlin Alinya Lawson, a scientific officer at the research station who’s been coming to Heron since she was an undergraduate, little more than a decade ago, is devastated by what’s happened on the island. 

“There’s no sugar-coating the fact that this is by far the worst that Heron has ever bleached in recorded history, as far as we know, which is terrifying to see and hear about,” she says. “So, yes, there’s no denying that it’s been bad. But reports that refer to Heron as a ‘graveyard’ and that it ‘looks like a bomb went off’, are not true.” And neither are reports claiming the situation is not recoverable. 

“Yes, it was sad, so sad,” Chris agrees with resignation as he recalls the scenes he saw beneath the water on his first day back at the island in 2024. “But I saw more live coral than expected and that means there is hope.” 

a snorkeller amongst bleached coral
bleached coral
Bleaching on One Tree Island in the summer of 2023–24 was widespread on the shallow reef flat, where you’d expect it to occur, but it was also extensive in deeper water, recorded down to at least 18m, which is normally something of a refuge area from the heated water that drives these events.

Because many corals are “broadcast spawners”, meaning they reproduce externally by sending massive numbers of eggs and sperm into the water column to mix with each other, dead reefs can become reinvigorated and reseeded by the fertilised eggs that float past on currents from surviving coral colonies. That means any glimmer of life on a dead or dying reef is a sign of hope for a future recovery. In fact, research indicates that in good conditions – when temperatures remain at consistently average levels – a reef destroyed by a bleaching event can be making a good comeback within a decade or so.

Chris points out places like Heron are not only crucial as research centres offering clues to how reefs might survive in a warming world, but they’re also important for tourism and education. “People ask me often if I think tourism should stop and I say absolutely not, because tourism is an important way to get ordinary people to truly experience the reef,” Chris says. “You want people to know what’s going on – to see live corals, dead corals and corals in ‘intensive care’. And you want them to see that there are still turtles and manta rays and lots of fish swimming around – that there is still
beauty there.”

Related: A quick guide to help you understand coral bleaching

Signs of stress

To appreciate what happens when a reef bleaches, it’s necessary to first understand that most corals are interconnected networks of tiny animals called polyps. Importantly, the survival of these relies on a complex symbiotic relationship they have with tiny plant-like single-celled algae called zooxanthellae. In hard corals, polyps excrete protective external cup-like skeletons of calcium carbonate in which their soft bodies sit. It’s these “homes” that can survive for millennia – long after the soft-bodied animals that construct them have died – and ultimately build the foundations of massive living reefs, like the GBR. 

Polyps gain a small portion of the nutrition they need to fuel their reef-building capacity by eating tiny animals drifting past in the water, snaring them with stinging cells called nematocysts. But the main source of food for coral polyps is carbohydrates, produced by the zooxanthellae living within their tissues. 

These tiny plants get a safe place to live, and the corals benefit from the relationship by having a steady source of food, which is produced by the zooxanthellae via the process of photosynthesis (as it is in most plants). And photosynthesis, of course, needs sunlight, which is why coral reefs exist mostly in clear shallow waters through which sunlight can penetrate. 

coral at One Tree Island
Three stages of coral stress occur simultaneously at this One Tree Island lagoon site – fluorescing, bleached and dead.

It’s the breakdown in this coral–zooxanthellae relationship that’s at the heart of bleaching events. When seawater temperature rises higher than corals are accustomed to, they become stressed and that brings about a series of tell-tale responses that ultimately see them expelling their zooxanthellae. Perhaps the earliest sign this is likely to occur is an increase in mucus in the water. Corals naturally produce mucus that usually sloughs off as part of a normal process. “When the coral is really stressed, it can produce more mucus,” Steph says, explaining that the water around a heat-stressed reef can be filled with sticky, snot-like material with an unpleasant smell. 

Another sign corals have been stressed by elevated water temperature is that they’ll fluoresce, hence descriptions of coral bleaching sometimes being a deceptively attractive process. “I call it the most beautiful disaster,” Chris says, explaining that fluorescing is a sign that corals are in need of “intensive care”.

a stressed coral fluorescing
Fluorescing can begin in corals about two weeks after heat stress. These vivid colours benefit coral by acting like a sunscreen, protecting delicate coral tissue, but they also function like a beacon for zooxanthellae, helping the algae return and recolonise coral tissue.

If the water temperature continues rising, corals eventually end up expelling their zooxanthellae permanently and that leads to another bizarrely stunning stage of coral ill-health – a bleached reef. But a bleached reef isn’t necessarily a dead reef: if temperatures return to normal, zooxanthellae can re-enter coral tissue and corals can bounce back.

Unfortunately, the temperatures recorded around OTI and elsewhere along the GBR during this most recent event have been so high and persisted for such extended periods of time that there has not only been much bleaching, but also widespread coral death. After they die, reefs quickly become covered in filamentous algae – devoid of the little fish and other life forms that usually live over, under and throughout them – and they can soon begin to look like lifeless wastelands.

Saving what we can

The rigorous long-term management of the GBR as a marine park will be crucial to how well it can bounce back from the recent disaster, and it’s GBRMPA that’s responsible for overseeing that. “I think the challenging part is to be able to see the reef for the coral,” says the authority’s chief scientist, Dr Roger Beeden, alluding to the fact that the reef will survive, even though individual patches of coral might be lost or transformed. “Even when we have these really substantial impacts, we have to look past and see how the reef is faring overall, and I think that’s where there’s some realistic hope.” 

Of course, it’s now undeniable that the world’s reefs – including the GBR – are taking a pounding from climate change. “The IPCC [Intergovernmental Panel on Climate Change] reports that, because of their sensitivity to temperature and ocean acidification and the increasing frequency of severe storms, the world’s coral reefs will not fare well as we increasingly go down the path of a changing climate. The impacts we’re seeing now are the steps along the path towards that prognosis,” Roger says. “But I think it’s important to recognise that there is actually a lot of action going on in terms of the mitigation space. Is it as much as many of us would like? Probably not. But at the same time, there is action happening.”

PhD students Isabella Marrable and Tony Hsu, in One Tree Island’s outdoor wetlab, are interested in how bleaching events affect butterfly fish, which feed almost exclusively on coral polyps.

Ultimately, in the long term, saving the GBR comes down to fighting for what we can but living with what we save, because the GBR is already showing signs of transitioning. “I think that’s unquestionable,” Roger says. “But what exactly will that look like over time? There are 450 species of hard corals that build the reef framework, and there’s an inherent adaptability within many of those species.” 

In coming decades it’s likely there’ll be changes in species composition, and changes in the ranges where species occur. Already these sorts of things have begun to play out. Recovery data from previous bleaching events are showing that early recovery on reefs is being driven by fast-growing, often short-lived, species in the genus Acropora – like the opportunistic weedy plant species that are the first to move into a disturbed terrestrial site.

“It’s a bit like early regrowth after bushfires,” Roger says. “And it’s not that those species are in any way better or worse than others, it’s just that we’ll get more of them because we’re having these more frequent disturbances. The future looks quite different from what it has been in the last few decades.”

An aerial view of the great barrier reef
One Tree is a misnomer. There is no one tree, but instead this isle of emerald green is covered in fleshy shrubs of beach cabbage, velvet soldier bush and small groves of flowering Pisonia.

The approach underpinning GBRMPA’s management of the reef is to keep it as healthy and vibrant as it can be, while the world continues to work at bringing down the atmospheric emissions driving climate change. It’s the same mentality that pervades the many research organisations and non-government conservation bodies working along the reef, and the tourism sector that so often operates beside them.

“There are some very tangible reasons why we have realistic hope,” Roger says. “For example, we’re very fortunate in Australia that we have a strong Protected Area Network [on the GBR]. A third of the marine park has full protection from any kind of extractive use like fishing. And we know that’s deeply important because even though the [GBR] is 3000 individual reefs, they are all connected, and that Finding Nemo story is real in terms of the water-based connections.

“Understanding those connections has also enabled us to target major new coral protection interventions like COTS control that directly supports recovery of reefs following other impacts such as severe tropical cyclones and increasingly frequent coral-bleaching events.” 

Related: What it will take to rescue the Great Barrier Reef

Australia’s stewardship

USyd marine ecologist Dr John Turnbull has been surveying reef communities and structure for more than a decade. The information he’s gathering now will ultimately help show how coral ecosystems respond to the recent bleaching event and that should assist in the management of heat-affected coral habitats in the future. But John is also heavily interested in the concept of stewardship and its influence on the sustainability of marine and coastal ecosystems. It’s clear there’s a lot of institutional stewardship at work to protect the reef, through GBRMPA and the many research organisations committing massive numbers of people and infrastructure to reef research. 

However, John feels that it’s ultimately the stewardship that individual Australians feel for the reef that will be vital to how it survives into the future. “I don’t think people realise how much trouble the reef is in,” John says. “Some probably feel that it’s being overexaggerated. But the people experiencing it firsthand don’t feel that. None of us feel that it’s being overblown. In fact, we’re shocked at what we’re seeing.”

It’s important, he believes, that the wider Australian population understands this, that the perilous impact climate change is having on the GBR is very real and that it’s not just the coral that is being affected, but also the other organisms that rely on coral structures for food or shelter. “The flow-on effects for the wider marine community are massive,” John notes. “And I don’t think people have made that connection yet.”

a aerial view of the great barrier reef
Shallow patch reefs in the One Tree Island lagoon
Shallow patch reefs in the One Tree Island lagoon had little reprieve in the summer of 2023–24 from high temperatures, with water heated by warm currents but also elevated air temperatures.

As a researcher, he and his colleagues are refusing to give up on the GBR and they hope that message will inspire the wider Australian – even global – community. Scientists, for example, each spend weeks at a time on reef-based research stations, away from friends and family, often conducting fieldwork for more than eight hours a day and then processing results well into each night. And many hundreds of them are doing that, up and down the GBR.

“To me, part of the real positive story here is we [researchers] get up every day and, even though we know we’re going to go out there and see horrible things, we still go out and do it, because we’re trying to lay down a record of what’s happening,” John says. 

He explains that it’s not just a record for the sake of being a record, but to help understand what’s happening so that it can inform managerial actions. John is optimistic that the same sense of commitment and positivity can be embraced by the broader population and that Australians come to appreciate this is a problem that is country-wide – a case of national decline. “I think in terms of our identity; we Australians see ourselves as outdoor people – as coastal, or beach, or bush people,” he says, explaining that the future of the reef is personal for a lot of Australians. “Even though the vast majority of us live in cities, we still feel part of us is the bush and the reef and the beach. I think – unlike other things where we might say, ‘Well, that’s not my problem’ – we do feel like this is our problem. We do feel like we have a sense of ownership and responsibility for it. And while I’m studying the pure ecological side now, I always try to have a social ear to what’s happening, and I see people suddenly waking up to the fact that the reef is in peril.

“So, yes, we do feel the need to not lose it on our watch. It’s the same with other remote places, like Antarctica. People really care about it. They’d hate to think that it all melted on our watch and would see that as a personal failure, even though they may never actually go there.” 

And that, John says, is how he believes most Australians feel about the GBR.

Related: Little lives lost