By Kathy Graham
As coral growth rates decline around the world due to increases in temperature and atmospheric CO2, some reefs off the coast of Western Australia are trending in the opposite direction, a new study has found.
The findings are published in the latest issue of Science and add to the growing body of evidence that coral reefs are responding to global warming.
Until now, scientists have mostly measured and observed declines in coral growth rates on the Great Barrier Reef off eastern Australia, as well as reefs near Thailand and in the Red Sea.
Although exact causes are unknown, the main culprits appear to be rising ocean temperatures and, in particular, ocean acidification.
"There is concern among coral biologists that at some point during the past two decades we may have passed a tipping point in terms of climate change factors affecting recent coral growth rates," says lead author Dr Tim Cooper.
Cooper and colleagues at the Australian Institute of Marine Science, set out to assess whether there's been a similar decline in growth rates on Australia's western coral reefs, and how these might relate to known changes in sea surface temperature.
The study focused on six sampling locations in an area of around 1000 square kilometres extending from the Houtman Abrolhos Islands in the south to the Rowley Shaols in the north, 15 kilometres off the coast of Broome.
Using underwater drilling equipment, the team extracted cores from massive, long living corals called Porites. These grow approximately one centimetre every year in a process known as calcification, during which time a band forms as the ever-dividing coral polyp produces more skeleton.
"Because these corals can grow for hundreds of years, they represent an environmental archive that allows us to study this process of calcification through time," says Cooper.
Although some cores date back to the 18th century, the scientists focused on the last 110 years from 1900 to 2010, which was common to 70 per cent of the cores.
"The most surprising finding is we haven't found a consistent decline across our study area", says Cooper.
The two most southerly locations, where recent warming is greatest, show a significant increase in calcification rates.
This contrasts with the two most northerly locations in the Rowley Shoals, where there was no significant change in calcification rates and a much lower rate of warming.
Only one location showed a significant decline in calcification rates since 1900.
Cooper says these findings help improve scientists' understanding of the effects of rising CO2 levels on coral reefs around Australia and globally.
"Rapid warming of parts of the tropical oceans - and not acidification which would normally cause a decline in growth rate - appear to be what's driving coral calcification responses," he says.
"Some corals in some locations can keep up with these changes, while others are already showing that the temperature changes have exceeded optimal conditions for coral growth."
Professor John Pandolfi, President of the Australian Coral Reef Society, says this is yet another study showing definitively that temperature has a controlling effect on coral calcification rates.
"It represents another cautionary tale about the immediacy for understanding the role temperature plays in marine organisms and illustrates the need to study the relationship between various species' characteristics and sea surface temperature in order to prepare for and predict the effects of ongoing global warming."