Researchers examining the impact of climate change on coral reefs have found a way to predict which reefs are likely to survive bleaching episodes and which won't.
Coral bleaching is the most immediate threat to reefs from climate change. It is caused when ocean temperatures rise above the normal maximum summer temperatures, sometimes leading to widespread coral death.
A key unanswered question has been what dictates whether reefs can bounce back from such events or if they will die.
An international team of scientists, including Adjunct Senior Research Fellow Shaun Wilson of The University of Western Australia's Oceans Institute, found that five factors could predict if a reef was likely to recover from a bleaching event.
"Remarkably, the structural complexity of a reef, which includes its physical features and species composition, and the water depth before disturbance, predict recovery with 98 per cent confidence," lead author Dr Nicholas Graham said.
Dr Graham, from the ARC Centre of Excellence for Coral Reef Studies at James Cook University, said other factors were nutrient levels, the amount of grazing by fish, and survival of juvenile corals.
As part of the research, published this week in the journal Nature, scientists from Australia, the United Kingdom and France examined nearly 20 years of coral reef data gathered from the Seychelles in the Indian Ocean.
The data spanned an unprecedented coral bleaching event in 1998, in which 90 per cent of the country's corals across 21 reefs were lost. Of the reefs affected by the episode, 12 recovered while nine did not. The event had a significant impact on local fish populations, which changed substantially when reefs did not recover.
From their data the researchers identified thresholds for the factors that dictated whether reefs recovered or were further degraded and taken over by microalgae.
"Putting numbers on the threshold points at which reefs either recover or degrade helps predict reef futures under climate change," Dr Graham said.
Dr Wilson, who co-authored the study, said the findings were important for reef managers.
"The beauty of this study is that easily acquired measures of structural complexity and depth provide a means of predicting long term consequences of ocean warming events, enabling targeted management actions," he said.
Dr Graham said the way climate change was going, some coral reefs would simply not recover. But being able to predict which ones had the capacity to do so was important for mapping winners and losers and doing risk analysis for reefs. Careful management of areas with conditions which made them more likely to rebound gave them the best possible chance of long term survival.
"Reduction of local pressures that damage corals and diminished water quality will help to increase the proportion of reefs that can bounce back," he said.
The paper, entitled Predicting climate-driven regime shifts versus rebound potential in coral reefs, is by Nicholas AJ Graham, Simon Jennings, M Aaron MacNeil, David Mouillot and Shaun K Wilson.
Adjunct Senior Research Fellow Dr Shaun Wilson (UWA Oceans Institute) (+61 4) 00 121 175
Dr Nicholas Graham (ARC Centre of Excellence for Coral Reef Studies, James Cook University) (+44) 747 943 8914
David Stacey (UWA Media Manager) (+61 8) 6488 3229 / (+61 4) 32 637 716