31/08/2016 8:55 AM AEST | Updated 31/08/2016 8:57 AM AEST

Lucky Escape As Sydney's Coral Bleaching Event Heals Itself

No one predicted coral would be bleached so far south of the tropics and they also didn't realise how quickly it would recover.

Jean-Pierre Pieuchot
Sydney's harbour may hold secrets for the future of corals.

When coral bleaching was detected in Sydney Harbour's decidedly un-tropical waters this summer, scientists were shocked -- bleaching had not been recorded this far south and suddenly, 45 percent of sites were white.

Yet four months later, almost every single bleached site is showing signs of recovery.

The beaching was recorded after an unseasonably hot summer brought a current of very hot water down the eastern seaboard, decimating corals including the Great Barrier Reef.

Matthew Nitschke
The same Sydney Harbour coral three months apart

UTS Climate Change Cluster doctor Matthew Nitschke said Sydney corals showed themselves to be particularly robust.

"The widespread coral recovery observed as temperatures returned to normal suggests these corals may also have some unique characteristics that makes them incredibly hardy," Nitschke said.

"Their condition is even more impressive given that the recovery period also included a significant storm event for Sydney Harbour in June, which would have likely stirred up sediment and limited light availability for some time."

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.

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.

Macquarie University Quantitative Ecology and Evolution associate professor Joshua Madin said Sydney's corals could well hold the key to increasing resilience in the nations reefs.

These cooler places could be refuges for corals in the future when seas will be warmer."Joshua Madin

"Our goal is to investigate the characteristics of corals that determine who wins and who loses during extreme events," Madin said.

"The Sydney Harbour corals are teaching us a lot about what it takes to live in cooler climates, but also about the possibility of these cooler places as refuges for corals in the future when seas will be warmer."

Either way, researchers agree Sydney's coral bleaching event was a warning shot from nature.