One Tree Island isn’t an easy place to get to. Part of the Great Barrier Reef, there is often just one short window each day when the tides are high enough for a small boat to cross the rampart of treacherous coral that surrounds the island. Only then can you reach the remote research station it houses.
I was travelling with from the University of Queensland in Brisbane. Like me, she was making the arduous trip to the island, found 100 kilometres off the Australian mainland, to investigate the scale of a mass coral bleaching event unfolding in the waters around the highly protected coral cay.
When water temperatures are higher than normal, corals eject the algae that live inside their tissues and provide them with food, putting them at risk of starvation and death.
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In the weeks leading up to our visit, the water temperature in the region around the island had reached the mid-30s in Celsius (mid-90s in Fahrenheit) – the recommended warmth for a spa hot tub. Researchers were only able to work underwater for half an hour before feeling like they, too, were cooking.
The same story was unfolding up and down the 2300-kilometre-long Great Barrier Reef. This week, the Great Barrier Reef Marine Park Authority and the Australian Institute of Marine Science . Half recorded high or very high levels of coral bleaching, and high to extreme levels of bleaching were common in the southern region of the Marine Park, where One Tree Island is located.
The good news is that the marine heatwave appears to have peaked and the waters throughout the reef have begun to cool, but they are still much warmer than the long-term average. Now, researchers are set to assess the longer-term damage.
Bleaching isn’t necessarily lethal, but if the water doesn’t cool quickly enough in the coming weeks then the corals won’t be able to recover. Most alarming is that this is the seventh mass bleaching event since the late 1990s. żěè¶ĚĘÓƵs are increasingly asking how much the reef can take before recovery isn’t possible.
Explosion of colour
Our journey to One Tree Island had begun the day before with a ferry ride that took us 80 kilometres offshore from Gladstone on the Australian mainland to Heron Island Research Station.
This region is far enough south that, before now, cooler waters and good luck have largely shielded it from mass bleaching, says at the University of Queensland. The reefs here are also some of the most highly protected and well managed in the marine park, he says. At One Tree Island, for example, public access and fishing is illegal and all activities other than research are prohibited. The fact that reefs so far offshore, distant from pollution, exploitation and development, are bleaching is extremely significant, says Kininmonth.
Heinrich Breuer, who manages the One Tree Island Research Station, part of the University of Sydney, with his partner Ruby Holmes, picked up Ward and me in his boat from the jetty on Heron Island to cross the 20 kilometres towards One Tree Island. After about an hour, Breuer slowed the boat and we could see fields of coral on the sea floor rising up out of the depths as the ocean shallowed.
For about 100 metres, as we passed over the coral wall, the reef was just a few metres below us and, in the crystal-clear waters, I could see a mosaic of bright colour. Contrary to what many people believe, however, the Great Barrier Reef often under-delivers on colour when it is healthy. Naturally it is dominated by dull browns and greens, which are the pigments of the photosynthesising algae that live inside the corals. What I saw looked so bizarre and dissonant, like a jigsaw puzzle interspersed with the wrong-coloured pieces.
During a mass coral bleaching event, not all corals turn bright white. Counterintuitively, sometimes a bleached reef, like the one beneath me, can be exquisitely colourful. This is because when some corals are stressed, the healthy brown colours of the symbiotic algae are replaced by an explosion of psychedelic hues as the coral colonies increase the release of fluorescing pigments in response to warm water.
This is a defence mechanism for the corals: the pigments act like sunscreen to shield the remaining symbiotic algae, called zooxanthellae, which are vulnerable to bright light. “Having that pigment is a layer of protection for those remaining zooxanthellae, so it just gives them an extra chance at surviving,” says Ward.
Soon, Breuer had pulled the boat alongside the shore at the station. To go diving outside the reef, we had to transfer immediately to a smaller boat as there were only another couple of hours before the tide would be too low to cross the reef again.
Within a few minutes we were back on the open ocean, getting ready to roll backwards into the sea. Ward went in first and I followed. As soon as the bubbles cleared from my mask, the scale of this bleaching event was even clearer – everywhere we looked was dead, bleached or fluorescing coral.
We dived at another five sites and everywhere was the same. Even sea anemones, some of which have symbiotic algae in their bodies like coral, were bleached, though anemone fish were still swimming among their snow-white tentacles.
Exhausted and dismayed by the scale of coral stress we had observed, and with the tide falling fast, we returned to the research station, where we met , a researcher from the University of Sydney. Figueira had rushed to One Tree Island once the scale of the bleaching had become apparent to set up a monitoring programme to assess how the fish and corals responded.
“It’s pretty bad down here,” says Figueira. “We’re having trouble finding any spots where there is not a lot of bleaching. There’s a few taxa and species that seem OK, but in the vast majority of locations they’re either stressed or bleached, or in some cases there’s already mortality happening.”
Fluorescent coral
Just after dawn the next morning, Ward and I snorkelled at one of the most famous sites at One Tree Island, called the Gutter. As its name implies, it is a shallow channel that, at high tide, leads to the open ocean.
We caught a ride on the incoming tide and drifted along for a few hundred metres. There were reef sharks, bull rays, shovel nosed rays and turtles, as well as vast schools of mullet that took minutes to pass. But by far the most striking sight was the fluorescing, boulder-like coral outcrops, or bommies, that were so colourful they looked like giant hard-boiled sweets. Every shade imaginable was on display and it was clear that something extraordinary and frightening is unfolding. It was beautiful but also unnerving.
Later that day we headed back to Heron Island with Figueira and his students, breaking the journey by diving at another five sites. Even in deeper water and at sites where the current was so strong we couldn’t swim against it, there was no respite from the bleaching and fluorescing. Figueira used an underwater scooter to cover greater distances, but couldn’t find any unscathed areas.
Emerging from one dive, he looked shocked at the scale of the bleaching in an environment that has the highest levels of marine park protection. Once the warming threat gets high enough, there are no management and conservation tools that can save the coral – global warming will just overwhelm it, he says.
Back on Heron Island, there was one more place that Kininmonth wanted me to see. Blue Pools is one of the island’s most famous dive sites and parts of it were 100 per cent bleached. “All colonies were bleached to some extent and some have died,” says Kininmonth.
“To see a whole reef going from being pretty healthy, robust, greens and browns, and then to see it go into this bright white and blue, fluorescent image is amazing,” he says. “A month and a half ago that was a comfortably healthy site.”
Kininmonth says the critical thing for the reef now is time. The corals need time to recover and recolonise the reefs. “Another hot summer would be a disaster,” he says.
