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The Great Barrier Reef Hasn't Bounced Back From Coral Bleaching

After coral bleaching comes the algae, disease and predators.

26/10/2016 10:18 AM AEDT | Updated 26/10/2016 3:38 PM AEDT
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To the untrained eye, this footage may look like a fly-over of Queensland's stunning Great Barrier Reef. Yet researchers see that the colours aren't right. This isn't a healthy system, it's a lot of bleached coral after last year's unprecedented coral bleaching event.

And summer's warm currents are coming again.

Teams of researchers from the ARC Centre of Excellence for Coral Reef Studies at James Cook University surveyed the entirety of the Great Barrier Reef during the extreme underwater heatwave of 2015/2016 and now they've returned.

What is coral bleaching?

Coral hosts tiny algae in its tissues called zooxanthellae that produce its food as well as its colour.

When waters become too hot or cold, the coral becomes stressed and its metabolism and reproductive system break down.

Ho New / Reuters
Bleached coral loses its colourful algae.

At this point, it cannot process the oxygen created by the zooxanthellae, so it expels them.

If the temperature returns to normal, these tiny creatures can re-enter the tissues, or if most but not all algae have been expelled, the remaining few can keep the coral alive.

If the temperatures remain extreme and the sun continues shining, the coral dies.

There is no specific temperature threshold that tips coral into bleaching territory, rather, it's a cumulative stress.

While their initial reports showed the ghostly white of bleached coral, recent fly overs instead show murky colours. Researcher Greg Torda said it was not easy to see, but there was some hope.

"Six months after the peak bleaching, the corals now have either regained their algal symbionts and survived, or they have slowly starved to death without the nutrition the algae provide to them," Torda said.

"On the reefs we surveyed close to Lizard Island, the amount of live coral covering the reef has fallen from around 40 percent in March, to under 5 percent now."

One issue with surviving coral is they become food for animals that predate on coral.

ARC Centre of Excellence for Coral Reef Studies / Tane Sinclair
Crown of thornes sea stars munch on surviving coral.

Dr Andrew Hoey works on Lizard Island Research Station, surrounded by the reef, and said they were seeing a surviving coral being eaten.

"Snails that eat live coral are congregating on the survivors, and the weakened corals are more prone to disease," Hoey said.

"A lot of the survivors are in poor shape."

The good news is that reefs in the southern half of the reef were only lightly bleached and remain in good condition, with many individual corals able to bounce back.

Australian Marine Conservation Society Great Barrier Reef campaign manager Imogen Zethoven said she hoped this devastating event would spur stronger action on climate change.

"Let's be very clear, this continuing national environmental crisis has been caused by global warming, and the main contributor is the mining and burning of coal," Zethoven said.

The federal government is expected to report to the World Heritage Committee on the Reef's condition in December and Zethoven said others were seeking answers.

"The World Heritage Committee and the Australian public will no doubt want to know what our governments are doing about these findings, which show that just five percent of coral cover remains in reefs off Lizard Island in far-north Queensland.

"What more proof do we need for our governments to urgently move towards renewables, away from carbon polluting coal mines?"

As for the coming summer, the CSIRO and the Bureau of Meteorology's State of the Climate report was expected to be released on Thursday.

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