It’s too soon to tell if the heat-stressed corals of Australia’s Great Barrier Reef will survive their fifth mass bleaching in eight years, but the marine scientists who monitor the reef’s health are already despairing.
Species of coral typically resistant to bleaching are also struggling after the latest marine heatwave brought sea surface temperatures to a peak of 2.5°C above average.
Over the Australian summer, which ended in February, extreme levels of bleaching for the first time hit all three regions – northern, central and southern – of the world's largest coral reef system.
Unusually, the southern third of the reef, where waters are typically cooler, was severely damaged.
“This is definitely the worst bleaching event on record to ever hit the Great Barrier Reef,” Neil Cantin, of the Australian Institute of Marine Science, told RFI.
Cantin led a team that carried out aerial surveys in February and March along the 2,300-kilometre length of the Unesco heritage-listed site.
The results revealed that almost two-thirds had been struck.
Global catastrophe
Alarm over the health of the Great Barrier Reef comes as the warmest ocean temperatures in history, fuelled by El Nino, drive an ongoing mass bleaching of the world's corals.
It’s the fourth time such an event has happened globally, with more than 60 percent of corals suffering.
Bleaching occurs when heat-stressed corals expel the symbiotic algae (zooxanthellae) that live inside their tissues providing both food and colour.
While slow-growing varieties such as boulder and soft corals can better withstand thermal stress, others like branching and plate corals are far more sensitive.
“Some species can stay bleached for six to nine months,” Cantin says. “So it's going to take us another six months to fully understand the impact to the system and whether those bleached corals will now die.”
Research has suggested that even corals that do manage to regenerate are often weaker, less fertile and more vulnerable to diseases and future heatwaves.
Over the years coral bleaching has affected more than 98 percent of the Great Barrier Reef, home to more than 1,500 species of fish and hundreds of species of corals.
Corals 'already dying'
Scientists who have spent years studying the reef and diving its waters are grieving what’s been lost. They say things have never been this bad.
"I went out diving on one of the southern reefs at the end of March and it was quite confronting,” says Lissa Schindler, a marine ecologist with the Australian Marine Conservation Society.
Some of the corals had already started to die and were being carpeted by dense, thread-like algae – not the symbiotic kind.
In March you wouldn't usually expect to see any mortality yet, Schindler told RFI.
Evidence shows the stunning turquoise waters of the Great Barrier Reef, while still vibrant, are in tragic decline. There’s been a shift in the make-up of its coral reefs, with younger corals risking the system’s overall stability.
“You still see these amazing things,” Schindler says. “I took my kids out snorkelling and they saw turtles and sharks and they loved it... But for someone who knows how it should look – like myself and other scientists – we're feeling quite sad.”
Rainforests of the sea
A playground of biodiversity, coral reefs host a quarter of all ocean species and are nicknamed the rainforests of the sea. Reefs are also important for coastal protection and medical research.
Sick, white and imperilled corals are the first visual sign of how hotter oceans are hurting life beneath the surface.
For this reason, experts are comparing coral bleaching to climate-driven forest fires that are killing off the world’s biodiversity.
“The problem with coral reefs is that they're underwater,” Schindler says. “If this was a bushfire or on land, then it would be obvious.”
Danger listing
The UN is considering listing the Great Barrier Reef as a World Heritage site in danger – something the Australian government is working to avoid.
To merit world heritage listing, a site must have "outstanding universal value". Losing that status would be a blow for tourism, but a win for conservationists who want the ailing reef to be put on a global watchlist.
For the time being, it remains unclear how much of the reef will rebound.
Coral, an animal, can stay bleached and feed on plankton for a limited amount of time, explains marine scientist Cantin.
“That's what we're tracking now over the next few months and up to the next summer,” he says.
“We'll be performing more surveys to look at how much coral cover has been lost due to this bleaching event.”
