Great Barrier Reef under stress
Corals may not be able to adapt to warming as well as previously thought.
Warmer than normal ocean temperatures are causing detrimental coral bleaching, according to Queensland reef researchers.
A research paper published in PLOSONE by Central Queensland University's Dr Alison Jones and Dr Ray Berkelmans from the Australian Institute of Marine Sciences found that coral bleaching caused by warmer than normal ocean temperatures is impacting some corals more severely and for much longer than expected.
Earlier research pointed to the fact that to some extent some corals can acclimatise to ocean warming by changing their symbiont communities, a process known as shuffling. Given this, some researchers have suggested that coral reefs could move south as conditions on lower latitude reefs become too warm for them.
However, Dr Jones has questioned this possibility considering the stress caused to coral during a bleaching episode.
"Temperature stress obviously has a significant effect on coral calcification, retarding it by up to 50% for 18 months afterwards," Dr Jones said. "This has implications for the idea that coral reefs will adapt to warmer conditions and 'move south' - how can they if they are stressed and struggling to calcify?"
According to Dr Jones, local coral reefs should be considered 'stressed' for 18 months after a warmer than normal summer temperature-induced bleaching event - even if they don't look like they were affected.
Up until now, researchers and managers thought that corals recovered within a few months after a bleaching episode because they regained their colouration as they regained their algae. They believe that corals that didn't look bleached were considered 'unstressed'.
Dr Jones’ research shows this is not the case. "Even if corals host the thermally tolerant type D algae, and don't appear stressed during the event, there are medium-term sub-lethal impacts on their growth rate, just like in plants when severe hot conditions can affect productivity."
This has huge implications for reef managers because if corals have reduced growth rates following an event and these events become more frequent then the corals may have insufficient time to recover properly between them.
While the Keppel reefs have recovered magnificently between bleaching events four or more years apart so far, this may not be the case with bleaching events spaced one to two years apart.
Symbiont shuffling is definitely an acclimation benefit for reefs because corals that adopt thermally tolerant algae have a better chance of surviving temperature stress, but there is a trade off.
According to the report, once corals recover from the bleaching event and if they continue to host these thermally tolerant types, their growth rate will be ~30% less than it was before the event.
"Clearly, acclimation to climate change has costs for coral growth and we need to consider this in any future management of reefs because this means reduced regeneration potential."
CQ University's Director of the Centre for Environmental Management, Professor John Rolfe, said climate change would have a huge impact on both tourism and boating industries. He said reducing the activities on the reef and improving water quality will play an important role in helping the reef cope with climate change in the future.
