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Ocean acidification, brought on by anthropogenic climate change, is expected to negatively affect the growth of many marine creatures, including coral. Photo by Ethan Daniels/Alamy Stock Photo

Ocean Acidification: More Gloom than Doom?

Scientists’ emphasis on laboratory trials can give a distorted view of the threat of ocean acidification.

Authored by

by Evan Lubofsky

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When it comes to the threat of ocean acidification, laboratory studies paint a disturbing picture. Take corals, for example. After bathing in tanks of artificially acidified seawater, they begin to lose their skeleton-building abilities and eventually dissolve into dust. The corrosive power is convincing: even hard-nosed skeptics of climate change may concede that the acidification of the ocean is worth a watchful eye.

But in the wild, the picture can look vastly different. Corals in Palau, for example, have shocked scientists by not only tolerating, but thriving in water that is naturally acidic. These corals are comfortable in water with a pH level in line with those predicted to arrive elsewhere by the end of the century.

“What we saw in Palau was the exact opposite of what we expected,” says Hannah Barkley, a graduate student in the Massachusetts Institute of Technology-Woods Hole Oceanographic Institution Joint Program in Oceanography.

Ecological surprises like this help, to some degree, to balance out the doom and gloom nature of ocean acidification research. But they also suggest that laboratory experiments—which make up the bulk of scientific research on the topic—are only telling part of the story. This raises a key question: can sound predictions for the future state of the ocean be made based on experiments run in tanks?

A new paper published in the journal Global Change Biology argues they cannot. As Australian Institute of Marine Science coral reef ecologist and lead author Sven Uthicke says, laboratory studies are often conducted in highly controlled conditions, outside of ecological contexts. Thus they don’t take secondary effects on marine organisms into account. This means environmental factors or ecosystem dynamics that would balance out (or compound) the effects of acidification are being overlooked.

Coral dissolves in a laboratory trial in which it is exposed to acidified water. Photo by Anne Cohen, Woods Hole Oceanographic Institution

Such was the case in Papua New Guinea, where Uthicke studied the effect of acidity on sea urchins that live near volcanic vents, where pH levels are naturally low. Based on aquarium trials, sea urchins are normally considered highly susceptible to the threat of acidification. Yet Uthicke and his colleagues found that these urchins, which have had time to acclimatize to the conditions near the vent, grow at a healthy rate—even faster than at surrounding sites with more normal pH levels.

What got the urchins over the acidification hump, according to Uthicke, was the “secondary effect” of an increased food supply at the vent site: the same elevated carbon dioxide levels that caused the increased acidity also triggered the growth of algae, on which the spiny creatures could feast.  

Conclusions based singularly on lab studies, it seems, may not accurately reflect how organisms will fare in the wild. But are the dramatic results of lab trials exaggerating the risk of ocean acidification?

“For sure there have been what some may consider as exaggerations,” says Kevin Flynn, a marine biologist at Swansea University in Wales. “But at the same time, there have been instances where we have underestimated or misunderstood the potential impacts based on lab studies. In either case, it’s irrefutable that oceans are becoming more acidic. So we have to be very wary.”

Ian Enochs, a marine scientist at the University of Miami, agrees. He says it’s hard to argue with a huge body of evidence that suggests ecosystems will be severely impacted by ocean acidification.

“When looking at complex ecosystems, I wouldn’t say results from lab studies have been exaggerated. I think exaggeration implies agenda, and that’s not what science is,” he says.

Enochs, however, also sees gaps in lab studies. “If you do a lab study of just the adult species, it doesn’t factor in all the life history stages, such as larval settlement or juvenile stages, when organisms are often found to be especially susceptible to ocean acidification.”

Despite the limitations, many scientists consider lab studies an important and necessary first step in ocean acidification research. Natalie Hicks, a biogeochemist at the Scottish Association for Marine Science, sees the value: “Lab experiments help us mute natural background variation, so we can be certain the responses we measure are a result of a treatment or experimental condition.”

But acidification in the natural world doesn’t happen in isolation, and getting a complete, holistic picture of its effects requires fieldwork—something that has been hard for scientists to come by.  

“There just aren’t a lot of naturally acidified places to go out and study,” says Enochs, “and the handful of them out there can be difficult to get to. It can be a two-day boat ride just to get to some of these sites, but when you get there, they can provide rare glimpses into the future.”

As scientists scramble to find more low-pH field sites to run their experiments, lab studies, says Hicks, are starting to become more multilayered and complex.  

“The research trend has shifted to incorporating the interactions of multiple species and stressors in order to try to replicate the natural complexity of ecosystems,” she says. “The shift is a natural progression, and will make it much easier to scale up to ecosystem levels so we can get a better idea of how organisms will respond.”