In the face of rising atmospheric carbon dioxide, the Gulf of Maine is thought to be particularly vulnerable to ocean acidification. Its vulnerability has to do with temperature: The waters of the gulf are cold, and cold water dissolves carbon dioxide more easily than warmer water does. Increased carbon dioxide decreases the pH of the ocean (making it more acidic), a concern for the health of the region’s ecosystems as well as its lucrative shellfish industry.
But determining seawater chemistry is complicated. It requires advanced equipment and the assessment of complex physical, chemical, and biological processes. Until now, no long-term data existed to put individual measurements into context, so scientists did not know how acidity in the region’s waters was trending.
Using ocean chemistry recorded in algae, researchers have now constructed a nearly 100-year history of acidity (pH) in the region. The analysis, published in Scientific Reports, shows that ocean acidification, seen around the world, has been delayed in the gulf.
The Gulf of Maine is fed by three offshore water masses: icy, acidic northern waters from the Scotian Shelf and Labrador Current and warm, alkaline Gulf Stream waters. It’s also bordered by thousands of kilometers of shoreline to the west, and its estuaries and inshore waters receive significant riverine runoff.
The group expected to see pH fluctuate in the gulf, given the different factors affecting ocean chemistry and human-driven increases in atmospheric carbon dioxide, said Joseph Stewart, a geochemist from the University of Bristol in the United Kingdom and study coauthor. Data from 2011 to now, collected in Maine’s Casco Bay by a local nonprofit, show an increase in acidity in that coastal area. But that time frame is too short to determine long-term trends, according to the study authors.
Ocean Chemistry Recorded in Algae
Crustose coralline algae live for about 40 years in coastal areas of the Gulf of Maine, the southern limit of their range. These cold-loving algae encrust rocks and grow in seasonal increments, leaving growth bands akin to tree rings in their calcified skeletons. They are highly sensitive to changes in pH and serve as a record for past seawater carbon dioxide concentrations.
Using samples of the algae collected from several locations, the team reconstructed a timeline that spanned from 1920 to 2018.
“We’ve been measuring temperature for a long time, but we have not been measuring seawater pH for very long. It’s a very complicated, hard measurement.”
“We’ve been measuring temperature for a long time, but we have not been measuring seawater pH for very long. It’s a very complicated, hard measurement,” said Branwen Williams, a climate scientist at Claremont McKenna College in California and coauthor of the study. “So records like this are really valuable to get a sense of the variability that exists, particularly in these areas with people,” she said.
To the researchers’ surprise, the algae recorded a historic trend of relatively low pH in surface seawater, about 7.9, with a slight increase of 0.2 pH unit over the past 40 years. (On average, ocean water currently has a pH of around 8.1.) That move toward slightly more basic conditions was counterintuitive.
“We were somewhat surprised by that result, but then it made a lot of sense when we put it in the context of how temperature was changing and how nutrients were changing, and the timing of that change that had been previously documented in other papers,” Stewart said.
Starting around 2010, waters in the Gulf of Maine warmed dramatically. The change was driven by the decreasing influence of frigid northern water masses and the rise of Gulf Stream waters, which are not only warm but also alkaline. These waters seem to act as a buffer and delay the onset of ocean acidification.
Warming Waters
Ocean circulation–driven buffering effects will, at some point, reach their limits, researchers said. The ocean’s uptake of rising amounts of atmospheric carbon will persist, however, and leave the region’s ecosystems and economy vulnerable to the effects of acidification.
Ocean acidification presents one more challenge to the gulf’s coastal economy and its commercial fisheries, which stretch from Cape Cod to Nova Scotia. Ecosystems in the Gulf of Maine already face threats from disease, warming waters, habitat degradation, and invasive species. The added threat of acidification may push individual species past their ability to persist and redefine the biotic and abiotic factors contributing to those species’ ecosystems—a tipping point.
“It’s not just pH on its own that’s going to cause the ecosystem tipping point to occur, but a combination of pH and temperature, and both of those things are changing. The more data we have to understand the systems, all those different factors, the better,” said study coauthor Michèle LaVigne, an ocean scientist at Bowdoin College in Maine.
This and other studies provide insight into acidification trends, but the challenge of understanding and addressing competing factors influencing ocean pH feels intractable, said Damian Brady, an oceanographer at the University of Maine who was not involved in the study. “The dynamics are such that we have these countervailing forces all the time. We have these rises in total alkalinity from offshore source water, increases in temperature, while also, we as a species increase the carbon dioxide that goes in there,” he said. “It’s really complex.”
—Kimberly Hatfield, Science Writer
Citation: Hatfield, K. (2025), Warming Gulf of Maine buffers ocean acidification—for now,
Eos, 106, https://doi.org/10.1029/2025EO250239. Published on 3 July 2025.
Text © 2025. The authors.
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