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The Gulf of Maine—home to commercial fisheries for oysters, clams and mussels—has unexpectedly avoided an increase in seawater acidity, helping to preserve the health of its fisheries.

"Contrary to expectation," a team of scientists wrote in a paper in the journal Scientific Reports, we ... find that pH has increased (+ 0.2 pH units) over the past 40 years, despite concurrent rising atmospheric CO₂." (Determining the decades-long trend required measurements of boron isotopes within annual skeletal bands built by crustose coralline algae. More about that later.)

The Gulf's acidity levels are unexpected because atmospheric levels of carbon dioxide continue to climb. And because carbon dioxide is absorbed faster and easier into cold water, the frigid waters of the Gulf would be expected to take in more carbon. More carbon in water generally lowers pH and increases acidity.

"One goes down and the other goes up," said Alan Wanamaker, an Iowa State University professor of earth, atmosphere, and climate and a paper co-author. "That's what we've seen in the open ocean."

'A big mixing bowl'

Three main water masses that flow into the Gulf are responsible for the lower-than-expected acidity, according to the researchers. There are northern, cold, less salty and low alkaline/more acidic flows of Scotian Shelf Water at the surface and Labrador Slope Water at depth. And there's southern, warmer, more salty and higher alkaline/less acidic flows of Warm Slope Waters from the Gulf Stream.

The Gulf of Maine is "a big mixing bowl" for those water masses, Wanamaker said.

So far, a mix featuring Warm Slope Waters has been good for the Gulf's shellfisheries.

However, "This delayed onset of ocean acidification is cause for concern," wrote the researchers, including Joseph Stewart, the first author and a research fellow in the School of Earth Sciences at the University of Bristol in the United Kingdom.

"Once ocean circulation-driven buffering effects reach their limit, seawater pH decline may occur swiftly," they wrote. "This would profoundly harm shellfisheries and the broader Gulf of Maine ecosystem."

Algae skeletons as pH sensors

A major challenge for the researchers was the lack of a long history of seawater pH in the Gulf of Maine. Instruments had only been measuring pH levels for 13 years. How could they determine past levels and any trends?

Crustose coralline algae—"they're a lot like a coral," Wanamaker said—can live up to 50 years in the Gulf. They encrust submerged coastal rocks and create a hard skeleton-like structure made of calcium carbonate that includes annual growth rings.

Wanamaker, who directs the Stable Isotope Laboratory at Iowa State, has long used the oxygen isotopes stored in the annual rings of clam shells to piece together long histories of ocean conditions. Isotopes are atoms of an element that have the same number of protons within the nucleus but differing numbers of neutrons.

Scientists did similar work for the Gulf of Maine project, using boron isotope measurements within the bands of crustose coralline algae to construct Gulf pH records from 1920 to 2018.

Any drop of higher pH/lower acidity water from the Gulf Stream could lead to what the researchers call "tipping points" that create "the potential for abrupt ecosystem collapse."

"We worry about tipping points," said Wanamaker, who grew up in New England and has studied the Gulf of Maine since his doctoral studies at the University of Maine some 20 years ago. "We're not predicting when this might happen. But we're warning this might be an instance of a tipping point happening."

If conditions are tipping, "growers need to be thinking about potential changes in the Gulf," Wanamaker said. "And they need to start thinking about different shellfish or a diversified harvest. That might be important in these times."