Hakai Magazine

Coastal science and societies

An underwater fumarole releasing bubbles of gas
Environments with high levels of acidity, such as those around volcanic vents, provide a natural laboratory to examine the effects of ocean acidification on marine ecosystems. Photo by Matthew Oldfield Underwater Photography/Alamy Stock Photo

High-Acidity Reefs a “Desert” of Weed-Like Algae

Naturally acidic reefs paint an unflattering picture of the ocean ecosystems of the future.

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by Yao-Hua Law

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It’s now well established that the oceans are becoming more acidic as they absorb the huge amounts of carbon dioxide produced by the burning of fossil fuels. Using laboratory experiments, scientists have shown that acidification will hamper many marine organisms—from corals to oysters to fish. But the artificial settings used in most experiments mask more complex natural responses. It’s possible, even likely, that many of the consequences of ocean acidification discovered in the lab will not play out the same way in the wild.

An alternative to laboratory experiments is to study sites with naturally high acidity, such as underwater volcanic vents, which are rich in carbon dioxide. In a new study, University of Adelaide biologist Sean Connell compared how communities of algae and herbivores interact at four rocky New Zealand reefs. Two of the reefs have volcanic vents with acidity levels similar to what is expected for much of the ocean by the end of the century given ongoing acidification. The other reefs have no volcanic vents and served as a stand-in for the present-day ocean. Comparing and contrasting the four reefs shows how otherwise similar ecosystems behave in varying levels of acidity.

The researchers were looking at a few key factors: the abundance of kelp and turf algae—a weed-like, mat-forming mélange of algae and cyanobacteria—and the prevalence of two herbivores: urchins and sea snails. They also measured how quickly the algae grew and how voraciously the herbivores fed. The results showed striking differences between the two types of reefs, which could indicate how marine ecosystems will change as the oceans become more acidic.

At the carbon-dioxide-rich volcanic vents, kelp became less common, while the weed-like turf algae bloomed. Turf algae exploited the abundant carbon dioxide near volcanic vents and grew three times faster than at the vent-free sites.

Urchins, like most marine organisms that use calcium carbonate to build shells or other body parts, suffered in the acidic water. There were 90 percent fewer urchins near the volcanic vents than on the vent-free reefs. The urchins that did brave the acidic water ate 80 percent less food. Connell thinks the stress of the acidity affected the urchins’ metabolic rates. Unexpectedly, snails, which also have calcium carbonate shells, seemed less affected than their urchin neighbors—they were far more common and far hungrier on the volcanic vents. But even their appetite was insufficient to rein in the turf algae. Uncontrolled, turf algae displace kelp and reduce the biodiversity of the reef, morphing it into what Connell describes as a “desert of turf-forming algae.”

Malcolm McCulloch, a coral reef expert at the University of Western Australia, appreciates the value of conducting experiments in the field under natural conditions. But just as laboratory trials have limitations, so too do field studies.

McCulloch, who was not part of the study, argues that volcanic vents are not closed systems; seawater and organisms move freely in and out of the sites. He says it’s possible the differences between the reef ecosystems might be chalked up to some factor other than acidity.

Another possible caveat is time. Communities living near volcanic vents have evolved in these acidic conditions. Marine creatures will only have decades to respond to anthropogenic ocean acidification.

“The big question is whether organisms can adapt and keep up with the rate at which humans are changing their environment,” says Nyssa Silbiger, a marine biologist at California State University, Northridge, who was not involved in the study.

To Connell, the takeaway of his study is that complex natural systems can tolerate change, but only up to a point. Beyond that, interactions that work to keep the ecosystem in balance—such as the ability of grazing herbivores to keep weedy algae in check—begin to fall short.

While this study predicts a grim future from ocean acidification, Connell sees it in a positive light. “It tells me that being able to maintain the complexity of nature and make sure we aren’t overfishing the herbivores gives us a great chance at stability.”