Scientists have long been concerned that decreasing ocean pH (increasing acidity) caused by increasing carbon dioxide emissions in the atmosphere could negatively impact coral reefs. Many laboratory experiments have demonstrated that increased ocean acidity slows down coral calcification, the process by which corals grow and build their hard structure. But far fewer studies have tested how increasing ocean acidity affects coral growth in the natural environment, where a multitude of additional factors such as light, temperature, and nutrients are also important.
A new paper based on research led by scientists at Scripps Institution of Oceanography at the University of California San Diego that was published this week in Science Advances found a surprising answer to this question. Scripps chemical oceanographer Andreas Andersson, his graduate student Travis Courtney, and an international team of collaborators from the Bermuda Institute for Ocean Sciences and Christian-Albrecht University in Germany, among others, were involved in the study funded by the National Science Foundation.
The researchers collected environmental data including temperature, light, and pH from two coral reefs in Bermuda over a period of two years. They simultaneously measured the calcification and growth rates of coral samples placed on each of the reefs, as well as seawater chemistry. The team also calculated reef-scale calcification for one of the two reefs studied. All of this information was then analyzed with structural equation models (SEM), which were used to simplify the relationships between different factors to show which environmental variable affected calcification the most.
“The biggest result from the SEM analysis is that temperature is the only environmental driver that has significant results for calcification by both coral species at both reef sites and for reef-scale calcification,” Courtney said. “It was the only significant environmental parameter for all calcification measurements that we looked at in this study.”
They found that in Bermuda, coral calcification was relatively insensitive to changes in the seawater pH, but very sensitive to changes in temperature. And the observed relationship between temperature and calcification was a positive one—as the seawater got warmer, coral growth sped up.
The positive effect of modest warming on coral calcification had been observed in many laboratory experiments. But, as Courtney explained, it comes with a caveat: if the warming is too much or too fast, corals quickly reach a tipping point.
“Then there's a very sharp downturn, which is usually somewhere around the maximum average summer temperature—just a little warmer than what they’re used to—where corals have a very quick downturn in how fast they can grow,” Courtney said. “About another degree past that maximum summer temperature is often when bleaching can happen. The coral is stressed, and it starts growing slower, and then stress mechanisms trigger bleaching and it loses its symbiotic algae. If that bleaching stress is severe enough for long enough, that's when you can have coral mortality.”
Many coral reefs around the world are already beginning to exceed that point of beneficial warming in the summer. The Northern Great Barrier Reef in Australia, for example, had a massive bleaching event last spring when seawater temperatures climbed to two degrees above the average summer maximum and stayed at that peak for eight weeks. But because Bermuda reefs are so far north, in relatively cooler waters, summer temperatures haven’t yet reached the highs that would trigger mass bleaching events, and instead a warmer winter helps them grow more year-round.
“At the present time, Bermuda's coral reefs appear to be faring reasonably well with high rates of coral production and calcification that appears to be temporarily offsetting the impact of ocean acidification,” said Nicholas Bates, the director of the Bermuda Institute for Ocean Sciences and a co-author on the paper.
“We saw that as the temperature got warmer, there wasn't a point in the data where it got too warm and calcification started to slow down,” Courtney said. “We also didn't observe significant coral bleaching in the study, so in Bermuda the thermal maximum, or the highest temperature at which coral can grow, likely wasn't exceeded during the two-year study.”
But Andersson and Courtney were quick to point out that this could change in the very near future if warming trends continue. If the rate of ocean warming is too fast, warmer water will soon be dangerous instead of helpful for Bermuda corals.
“If the rate of CO2 emissions continue to increase at their current rate, it's going too fast, and there will be detrimental effects,” Andersson said. “We see that by 2040 or even earlier there would probably be extensive bleaching.”
Additionally, Andersson said that just because so far the positive effect of temperature on the Bermuda reefs overwhelmed the negative effect of acidification, it doesn’t mean acidification isn’t a problem.
“This doesn't mean that the corals are not negatively affected by pH,” Andersson said. “It just means that the positive effect of the warming may mask any negative effects from pH when you're within the natural variability of temperature.”
But, he said that based on model projections, they found that if countries adhere to the Paris agreement and are able keep global warming below one and a half to two degrees Celsius, corals in Bermuda could actually potentially increase their calcification rates under that scenario.
“It really highlights that it makes sense to reduce the CO2 emissions to follow this lower emission pathway. It makes a big difference,” Andersson said. “Some corals in Bermuda may actually grow faster if we manage to do that.”
Andersson said there are some places in the world where it is likely already too late, the water is too warm, and many corals can’t be saved. The planet has already lost fifty percent of its coral reefs compared to pre-industrial times, not only from over-heating but also pollution, over-fishing, disease, and coastal development. But Andersson said it’s not too late to save the remaining fifty percent.
“If we're able to follow the Paris agreement, there's going to be a higher proportion of corals globally than if we don’t follow the agreement,” Andersson said. “There's no doubt about that.”
This research was supported by the National Science Foundation, grants OCE 09-28406 and OCE 14-16518, and in part by a National Science Foundation Graduate Research Fellowship awarded to Travis Courtney.
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