(AU) Reefs At WA's Rowley Shoals Make Surprise Recovery From 'Significant' Coral Bleaching

ABC News - Ben Collins

Scientists inspect healthy coral on the slope of Clerke Reef at the Rowley Shoals. (Supplied: Department of Biodiversity, Conservation and Attractions)

Coral Comeback Widespread coral deaths were feared after 60 per cent of coral in areas of the Rowley Shoals bleached in April The latest survey has found much of the bleached coral is recovering The Rowley Shoals are a chain of three coral atolls 300km off Broome, Western Australia

It was a depressing, if expected inevitability when Western Australia's Rowley Shoals showed the first signs of mass coral bleaching earlier this year, but a follow-up survey has found a remarkable recovery looks likely to preserve the reef's near-pristine health — at least for now.

Tom Holmes, the marine monitoring coordinator at the WA Department of Biodiversity, Conservation and Attractions, said that while his team was still processing the data, it appeared the coral had pulled off an "amazing" return towards health over the past six months.

"We were expecting to see widespread mortality, and we just didn't see it … which is a really amazing thing," Dr Holmes said.

Dr Tom Holmes prepares to dive in the ocean off Western Australia. (Supplied: DBCA)

The survey was a follow-up to one conducted in April that found as much as 60 per cent of corals on some Rowley Shoals reefs had bleached after the most widespread marine heatwave since reliable satellite monitoring began in 1993.

It has long been known that high sea temperatures cause coral bleaching which can kill coral — as seen by the devastation of the Great Barrier Reef off the Queensland coast — but what is less well known is that bleached corals do not die immediately.

"So when a coral bleaches, it's actually just a sign of initial stress," Dr Holmes said.

"Tiny little microscopic algae that live in the coral are expelled by the corals themselves, so the actual coral animal underneath is still alive."

However, corals rely on these microscopic algae as a food source and cannot survive for long without them.

"If that stress continues for a long time and those corals remain white, then it can lead to mortality," Dr Holmes said.

"But there are some cases of bleaching around the world where … that stress hasn't continued for a long time, and the corals have been able to take that algae back in from the water."

Dr Holmes believes that vital time gap between bleaching and dying created a chance for the reefs to recover at the Rowley Shoals, a chain of three coral atolls 300 kilometres off Broome on the edge of Australia's continental shelf.

Coral bleaching observed on the Rowley Shoals' Clerke Reef slope in April 2020. (Supplied: Chris Nutt, DBCA)

Marine heatwave

Last summer, as bushfires raged across Australia's east coast, an unprecedented marine heatwave enveloped the west coast, reaching from South Australia all the way up to the Kimberley in northern Western Australia.

The high seawater temperatures killed crabs, abalone and other molluscs in the south-west, and killed fish and oysters and caused widespread coral bleaching in north-west WA.

The Australian Institute of Marine Science teamed up with WA's Department of Biodiversity, Conservation and Attractions to survey the effects of the marine heatwave on offshore reefs, including those at the Rowley Shoals.

"It was actually the most significant bleaching event we've ever recorded out there," Dr Holmes said.

"Based on the original images that we were getting back in April, we were really quite worried at the extent of the bleaching."

Divers conduct coral and fish surveys at the Imperieuse Reef, the largest in the Rowley Shoals Marine Park. (ABC Kimberley/Supplied

Natural protection

The devastating effect of mass bleaching at the Great Barrier Reef is well known but many people may not be aware of similar catastrophes that have occurred in remote reefs off WA.

In 2016, scientists documented bleaching of 60-90 per cent of corals at Scott and Seringapatam reefs, about 400 kilometres north-west of Broome.

But something different was occurring in the Rowley Shoals further to the south.

"This is a reef that many of us hold in high esteem and I guess it has a special place in our hearts," Dr Holmes said.

"It's one of the only places in WA, and only one of the places in Australia, where we've had long-term stability in coral cover and to date it's been largely free of widescale bleaching events."

A survey boat floats in one of the coral lagoons of the Rowley Shoals. (Supplied: DBCA)

The ocean currents off north-west Australia seemed to direct the increasingly warm summer currents away from the Rowley Shoals, until April this year.

"This particular event was not predicted and it was a little bit unusual, but what happened was that warmer water moved a little bit further south than we've previously seen it," Dr Holmes said.

But as the change in water currents came at the end of summer, the unusually warm sea temperatures did not bathe the Rowley Shoals for long.

"We had this sudden temperature peak, which is when the bleaching occurred," Dr Holmes said. "But then the water temperatures dropped down fairly quickly."

This likely would have reduced the stress on the coral and enabled it to recover.

Calm before long-term storm

A large school of humphead parrotfish on the slope of Clerke Lagoon. (Supplied: DBCA)

The relief that perhaps only 10 per cent of the coral at the Rowley Shoals has been killed, rather than the feared 60 per cent, is tempered by the knowledge that worse is almost certain to come.

"We feel like we may have dodged a little bit of a bullet in terms of a widespread bleaching event," Dr Holmes said.

"But looking forward, there's a degree of pessimism there I think … it's not great."

James Gilmour, from the Australian Institute of Marine Science, agrees that reefs in the Rowley Shoals appear to have escaped widespread coral mortality this time, but he fears they may not fare as well over the next 10 years.

"With ongoing climate change and ocean warming, we expect repeated bleaching events in the coming decade, and perhaps severe mass bleaching," Dr Gilmour said.

"Climate change will also result in more severe cyclones, and cyclones are the most common disturbance to coral communities at the Rowley Shoals."

Documenting the demise of Australia's once pristine coral reefs is the downside of what might at first look like a dream job for Dr Holmes as he dives at the Rowley Shoals.

"To the tour operators who go out there, to the fishermen who utilise it, to the scientists who study it, it's quite devastating to see it," Dr Holmes said.

"If we don't deal with the climate-change issue and the warming waters, then we're going to have some serious issues here."

Links

• As Australia bakes, a marine heatwave of historic scale is killing sea life along the west coast
• Coral bleaching leaves reefs off WA coast badly damaged
• Big Parts Of The Great Barrier Reef Are Dying
• More Than Half Of Remote Reefs In Coral Sea Marine Park Suffered Extreme Bleaching
• (AU) Great Barrier Reef Found To Be Coral Bleached From North To South For First Time
• Great Barrier Reef Likely To Be Hit With Another Mass Bleaching This Summer, Forecast Shows
• Bleaching Of The Great Barrier Reef Could Happen Every Two Years, Report Finds
• Study Of Reefs Reveals Benefits Of Keeping Global Warming In Check
• 2019 Was A Record Year For Ocean Temperatures, Data Show
• 'Death Blow': Corals, Algae Don't Acclimatise To More Acidic Seas
• Deep-Water Corals Also Bleaching Badly
• Study Of Reefs Reveals Benefits Of Keeping Global Warming In Check
• Coral Reefs ‘Will Be Overwhelmed By Rising Oceans’
• Saving The World’s Coral To Avert A Wipeout Of Irreversible Costs
• Global Warming’s Toll On Coral Reefs: As If They’re ‘Ravaged By War’