The Great Barrier Reef may be in hot water for the second year in a row.
Only one year after the worst bleaching on record, it’s happening again. The extent and severity of the bleaching this year remains to be seen—scientists are out on the Reef this week, surveying the damage from small planes.
Some corals can bounce back but prolonged bleaching can kill the coral. The concern now is that those corals bleached in 2016 could be hit again before they have had enough time to even start to recover. Even for those species that are good colonisers and fast growing, recovery can take 10–15 years. For long-lived corals, some of which can be over 100 years’ old, recovery can take decades.
Major bleachings in the past have been years apart (1998, 2002, 2016), allowing time for the corals to recover. Another bleaching on the back of 2016 would be disastrous and unprecedented.
Why do some reefs seem to be more susceptible to bleaching than others?
In a new study, the results of which are published this week in Nature, scientists show that it all comes down to the temperature of the water. When they overlaid the geographical spread, or footprint, of warmer-than-normal water onto the footprint of the three major bleachings of 1998, 2002 and 2016, it was evident that those reefs that were repeatedly exposed to the warmer water were those that suffered the most severe bleaching.
The problem of warmer water is not peculiar to the Great Barrier Reef—it applies to tropical and sub-tropical reefs all over the world.
On the west coast of Australia, CSIRO marine ecologist Dr Russ Babcock has spent years monitoring the extensive area of reefs along the Pilbara coast, including the World Heritage listed Ningaloo Coast. One of the 46 co-authors of the Nature paper, he has contributed bleaching data collected under the Pilbara Marine Conservation Partnership, is funded by Gorgon Barrow Island Net Conservation Benefits Fund and administered by the by the Western Australian Department of Parks and Wildlife.
“At low tide, there are more than 1000 islands along the Pilbara coast and we have been going out every year surveying everything that’s growing on those reefs,” says Dr Babcock. “We SCUBA dive, or snorkel if it’s shallow, taking transects, taking photos, and assessing the condition of the corals, whether it’s bleached, whether it’s dead. We are building a picture of what’s changed over time.”
Scientists in Western Australia are on alert this year, he says. “There are predictions for warmer water from Ningaloo down to Shark Bay and further south. So we’ll be going out again in May, to established sites we’ve been surveying, in some places for more than 10 years.”
By May, the cyclone season has peaked. “It’s also when you are most likely to see mortality, after the peak of summer. It usually takes 6–8 weeks of warmer temperatures to stress the corals before they bleach and die.”
While 2016 was disastrous for the Great Barrier Reef, events can march to a different drum on the west coast.
“We had bleaching in 2016 from the Kimberley up to East Timor but no bleaching south of that,” says Dr Babcock.
“Even if we had bleaching in 2016, we wouldn’t have seen much change in most parts of the Pilbara because much of the coral was already dead. We had severe bleaching in 2011 and 2013.”
The difference between the east and west coasts is related to how the El Niño–Southern Oscillation (ENSO) affects sea surface temperatures in a given year, he explains:
“In a La Niña year, such as 2010–11, it tends to be warmer on the west coast because of the way the water shifts. Temperatures have also increased in the oceans globally, so the spikes are even higher. But we also get bleaching when there’s no particularly strong ENSO signal—there’s just too much warm water around. These types of bleachings look like they are becoming more the status quo than the exception.
“Our records don’t go back as far as those of the Great Barrier Reef but there have been previous bleachings of corals in the west and, with the two recent bleachings in 2011 and 2013, it seems we are on a similar trajectory.”
Protecting coral reefs through zoning and managing pollution and over-fishing may help the corals bounce back but they will not prevent bleaching, the study found..
“We’re doing everything we can at the local scale to allow reefs to recover,” says Dr Babcock, “but with these large-scale bleachings coming more frequently, they don’t have time to recover. The real need is for urgent global action to reduce global warming and stem the rise in sea surface temperatures.”
Read the full Nature paper at “Global warming and recurrent mass bleaching of corals“, Nature, Volume:543,Pages:373–377, 16 March 2017. (DOI:doi:10.1038/nature21707).