Ensia

Is pH a red herring when it comes to ocean acidification?

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Courtesy of Ensia

New research points to saturation state, not pH, as the most pressing metric to track when it comes to shellfish survival.

In 2007, the owners of Whiskey Creek oyster hatchery on the Oregon coast lost almost all of their larvae — and had no idea why. The only clue was that the larval die-offs often occurred during intense upwelling events, when deep, acidic waters replace surface waters blown offshore. The next year, the hatchery owners turned to Burke Hales, a biogeochemist at Oregon State University in Corvallis, who dove into the ocean’s complex carbon chemistry.

Ocean acidification was clearly a growing concern. Globally, analysis of ocean pH — a common measure of acidity based on the number of hydrogen ions found in the water — shows that as more carbon dioxide from the atmosphere dissolves into the ocean, hydrogen-ion-releasing carbonic acid forms, lowering pH.

But Hales was never convinced that pH was the direct cause of the larval failure. Working with OSU marine ecologist George Waldbusser, he conducted laboratory experiments that found no clear pH response. It’s taken almost eight years, but the team now has the data to prove pH isn’t to blame.

Hales and Waldbusser developed an experimental method to decouple the effects of pH, dissolved CO2, and the saturation state of the shell-building mineral carbonate — essentially, the amount of the mineral dissolved in the ocean water relative to the maximum amount it can hold — to see which factor most impeded larval shell development. Using the new method, the team showed that survival rates plummet for larvae — both the Pacific oyster and a Mediterranean mussel — when the saturation state of carbonate dips too low, because their shells grow more slowly and are deformed.

That’s not to say that low pH doesn’t harm shellfish. It does, most significantly by altering the internal chemistry of adult shellfish. But saturation state–caused declines will occur decades to centuries ahead of any pH-induced declines because saturation state is more sensitive to increasing CO2 than is pH and several coastal regions in the Pacific Northwest are already dangerously close to the saturation state threshold for oysters and other bivalves. Essentially, saturation state matters most and it matters first, says the team. As at Whiskey Creek, a dip in saturation state, even for as little as two days, during a crucial stage of shell development is enough to lose an entire population.

Waldbusser and Hales worry that the disproportionate attention being paid to pH will hamper efforts to predict the most pressing biological impacts of acidification. Ongoing efforts to monitor ocean acidification are focused almost exclusively on pH. In fact, a $2 million ocean health XPRIZE, to be awarded later this year, is meant to encourage development of an affordable, accurate pH sensor to help improve monitoring.

Saturation state is far more difficult to quantify than pH: It must be calculated from other measures of carbonate chemistry, such as total dissolved carbonate and the partial pressure of carbon dioxide. And monitoring devices must be custom built. At roughly $50,000 each, the devices Hales builds to measure saturation state are at least three times more expensive than the most precise submersible pH instruments used in near-shore environments.

Spendy as they may be, devices able to offer comprehensive feedback on the entire carbonate system are what it took to pinpoint the true cause of the Whiskey Creek crash. And those measures will be crucial to determine the most appropriate actions necessary to prevent future ecological losses.

As Waldbusser says, “pH is important, but it doesn’t tell us the whole story.”

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