The List of Explanations for Ocean Acidification Keeps Getting Smaller

Tatoosh Island, Washington (photo from the Pfister Lab)

Tatoosh Island, Washington (photo from the Pfister Lab)

Over the past 20 years, Cathy Pfister and her husband Tim Wootton, both biologists in the Department of Ecology and Evolution, have been traveling to Tatoosh Island off the northwestern tip of Washington state to study the rich variety of plant and animal life in and around its coastal waters.  And while they have turned this tiny, rocky outcropping into a second home, they have also collected data that tell a troubling story about how climate change is affecting our oceans.

For years, they have been taking routine measurements of the ocean around the island, data about the temperature and chemical composition of the water that Pfister describes as the boring part of their field work. In 2008 this data became a little less boring when they saw that pH levels in the water were declining at a rate 10 times faster than what was expected.

When they published these initial findings in PNAS, they weren’t sure what was causing the rapid change, but the signs certainly pointed toward one culprit: increased carbon dioxide in the atmosphere. As humans burn more fossil fuels, more carbon dioxide goes into the air. The ocean absorbs at least a third of this carbon, which makes the water more acidic and lowers its pH.

Plants and animals depend on a certain pH level in the water. They can adapt if that level changes naturally, but not at such a rapid pace. The immediate impact of this increased acidity and lower pH is being seen already in shellfish. Animals like mussels and oysters have a hard time growing shells when the water is too acidic. Wootton said that when they published the first paper in 2008, some of the first people to contact them were oyster farmers who talked about the difficulty they’d been having growing oysters in the area.

At the time though, they couldn’t prove their hypothesis definitively, and in the meantime other scientists offered their own hypotheses, from changes in ocean currents to the influx of fresh water from increased rainfall. In the years since, Pfister and Wootton continued to collect data and publish studies about the changes happening around Tatoosh Island. For example, last year I spoke to them about how they had been able to measure carbon isotopes in mussel shells, which track the pH levels in the water. Shells collected in the past decade showed a steep decline in pH, while shells up to 1,000 years old contributed by the local Native American tribe don’t show the same changes. Now, in a new paper published in PLOS One, they show how their work continues to show declining pH as atmospheric carbon dioxide increases and casts doubts on alternate explanations.

Pfister said working toward an answer through such a process of elimination is frustrating. “We call it the ‘No Hypotheses are Supported’ paper,” she said. But whittling down other explanations for the rapid decline in pH makes them more certain about their own. “It really seems to be atmospheric carbon dioxide, and it’s doing something different than any of us understand it to be doing,” she said.

One of the popular alternate explanations for the low pH levels is that the circulation of the ocean has changed around Tatoosh Island, allowing deep water to come up to the surface. This could be due to systematic changes in currents or through upwelling, where winds pull surface water away from the shore, allowing the deep water to rise. But Pfister and Wootton could find no evidence of systematic changes in currents or wind patterns that would account for significant upwelling.

The deep water should also have much higher levels of nitrogen and other nutrients because plants and organisms that photosynthesize closer to the surface hadn’t processed it yet. They didn’t find evidence of this in the water either.

Wootton pointed out another key piece of evidence from their data.

“One of the biggest problems with a lot of these hypotheses is the notion that carbon dioxide must be produced by things that are breaking down organic material and respiring it into the water at these lower depths,” he said. “But that involves using up oxygen at the same time, and we don’t see any decline in oxygen with the change in carbon dioxide.”

This creates a problem for another hypothesis about the declining pH levels – that it must be caused by carbon leaching into the water from increased plant growth on land. The idea was that more carbon dioxide in the atmosphere made it easier for plants to grow. More plant life would lead to more organic matter dissolving into rivers in the area that led to the sea. But this would also result in decreased oxygen as organisms consumed that extra carbon.

The paper shoots down several other hypotheses, from the effects of increased fresh water input (rainfall records don’t support that) to eutrophication, or an increase in nutrients (which would also produce an accompanying decline in oxygen).

Pfister said that while she feels certain of their conclusion that the decline in pH is being caused by the ocean absorbing more carbon from the atmosphere, she hopes that more researchers will collect and analyze this kind of data, especially in coastal areas that are rich with animal and plant life.

For their part, Pfister said she and Wootton will continue tracking conditions on the tiny island they’ve made a career of studying.

“One of the advantages that we should use is that we have been working out there over 20 years,” she said. “We have data on how things have been fluctuating and changing over a long time, and we should be trying to see if changes in ocean chemistry have any explanatory power.”

Wootton, J., & Pfister, C. (2012). Carbon System Measurements and Potential Climatic Drivers at a Site of Rapidly Declining Ocean pH PLoS ONE, 7 (12) DOI: 10.1371/journal.pone.0053396

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