Coral and algae species subjected to more acidic seawater showed no acclimatisation to the new conditions for over a year, a new study has found, suggesting that vulnerable reefs may not be able adapt fast enough to cope with climate change.
With oceans absorbing about 22 million tonnes of carbon dioxide from the atmosphere a day, seas have already become about 30 per cent more acidic over the past two centuries.
Shell-forming creatures from oysters to types of plankton are increasingly at risk from the changes, which have been called the "evil twin" – along with higher temperatures – of climate change.
A team of scientists exposed four coral and two coralline, or calcifying algal, species to varying ocean acidity levels for a year at the Moorea reef in French Polynesia to test their responses.
They found "completely no change over a year", said Christopher Cornwall, a researcher based at Victoria University in Wellington, and an author of the paper published in Nature Climate Change on Tuesday.
"We expected that over the course of the year they would slowly get acclimatised," Dr Cornwall said. "In reality they displayed the same responses at the start and the end of the experiment."
Since some coral species are known to be more tolerant of lower pH water than others, they would be expected to become more dominant over time as ocean acidification increases – provided they can cope with the marine heatwaves that trigger bleaching events.
The stability of the reefs themselves, though, could be undermined if the calcifying algae become less productive, Dr Cornwall said.
"The reef itself will start to erode as all of those calcifying organisms are no longer producing that calcium carbonate," Dr Cornwall said. "Ocean acidification is kind of like the death blow after these warming events have been happening … there's nowhere to get refuge from that."
Ken Anthony, principal research scientist at the Australian Institute of Marine Science, said ocean acidification was a "more precise indicator than temperature" of what's happening in the biosphere as CO2 levels rise.
On current emissions trajectories, global ocean pH levels could drop to as low as 7.8, Dr Anthony said. Even at 8, all sorts of physiological changes can be expected, including entire ocean food chains being placed at risk.
"Even fish larvae's nervous systems get affected [by more acidic water]," he said. "Macro algae also grows faster so that suddenly weedy algae are much more successful in over-growing corals."
Dr Anthony, who was not one of the paper's authors, said he was "not surprised by these results", adding it "would have been wonderful" if adaptation had happened so fast.
He noted the study had focused on adult corals and algae, and future studies could be extended to look at juvenile and larvae "that are often more susceptible". Longer studies would also be helpful.
A separate study by AIMS researchers, meanwhile, has found the relatively pristine and remote coral reefs of Western Australia are increasingly being affected by heat stress and coral bleaching.
"For most (75 per cent) reef systems with long-term data (of five to 26 years), mean coral cover is currently at (or near) the lowest on record and a full recovery is unlikely if disturbances continue to intensify with climate change," the paper found.
Research should focus on identifying which reef systems are "least susceptible to future disturbances" and they should be preserved "through networks of protected areas", it said.