Published in Science: Australian researchers have found that Great Barrier Reef (GBR) corals were able to survive past bleaching events because they were exposed to a pattern of gradually warming waters in the lead up to each episode. However, this protective pattern is likely to be lost under near future climate change scenarios.
The researchers established that pre-stress periods in which corals are exposed to warmer waters act like practice runs that induce heat shock responses in the corals.
The team led by researchers from the ARC Centre of Excellence for Coral Reef Studies (Coral CoE) examined 27 years worth of satellite data for sea surface temperatures, previous coral bleaching events, and studied how corals responded to different seawater warming conditions.
They established that in three-quarters of stress events that occurred on the GBR in the past three decades 'practice runs' reduced the severity and mortality of corals.
But early evidence suggests that in the 2016 GBR bleaching event, some individual reefs not previously exposed to bleaching stress at all missed out on practice runs, suggesting that the damage to the corals on those reefs could be even greater.
And pre-stress conditions may disappear alltogether when seawater temperatures rise by as little as 0.5 ℃, which is predicted for the near future.
Therefore, if future climate change predictions eventuate, future summers bleaching events could not only occur more frequent, but with corals directly exposed to the stress events, they could become much more severe.
Different reefs on the GBR will lose their protective mechanism at different rates. The study recommends that reefs able to retain the 'practice run' of protective conditions prior to bleaching be given high priority for conservation efforts.
According to Associate Professor Bill Leggat, Coral CoE, understanding temperature pattens for different reefs, how these impact coral survival and their capacity to recover from bleaching, and how quickly changes in these patterns occur, will inform reef management.
In addition to researchers from the Coral CoE at James Cook University and the University of Queensland, the study involved a scientist from Macquarie University and collaborators in the US.
In a comment provided by the Australian Science Media Centre, Professor Terry Hughes, who is director of the ARC Centre of Excellence for Coral Reef Studies, said:
"The Great Barrier Reef (GBR) is currently experiencing the most severe coral bleaching event ever recorded. Aerial surveys of the entire Reef will be completed in the next few days by researchers from the ARC Centre of Excellence for Coral Reef Studies.
"The northern half of the GBR is severely damaged by bleaching, and on some individual reefs half of the bleached corals have already died. In the south, where bleaching is less severe, lightly bleached corals will regain their colour in the next few months as temperature drop. The underlying temperature of the GBR has risen steadily over the past 30 years due to global warming. As a consequence, the extra pulse of heat from recurrent El Niño events now triggers regional-scale bleaching events every few years. The first mass bleaching on the GBR was in 1998, the second in 2002, and this is the third and by far the most serious.
"The Ainsworth et al. study in Science shows that seasonal local weather affects the vulnerability of corals to bleaching during the peak temperatures of summer. This finding is directly relevant to the north-south pattern of bleaching we now see on the GBR. The southern section has been protected from bleaching by the cyclone that earlier passed over Fiji, when it brought cloud cover to the central and southern Queensland coast. Without that chance event, the entire GBR would have been even more damaged."