Climate: Study eyes regional patterns of ocean acidification

Gulf of Mexico appears more resistant to acidification threats

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The impacts of ocean acidification will vary from region to region. Bob Berwyn photo.

By Summit Voice

SUMMIT COUNTY — A 2007 sea voyage through the Gulf of Mexico, around Florida and up the eastern seaboard has increased understanding of how various coastal areas may respond to increased acidity. More than anything, the detailed research helps establish some baseline data against which future changes can be measured, and showed that some areas are more susceptible to higher concentrations of carbon.

The study, measuring levels of carbon dioxide and other forms of carbon in the ocean, was conducted by scientists from 11 institutions across the U.S. and was published in the journal Limnology and Oceanography.

“Before now, we haven’t had a very clear picture of acidification status on the east coast of the U.S.,” says Zhaohui ‘Aleck’ Wang, the study’s lead author and a chemical oceanographer at Woods Hole Oceanographic Institution. “It’s important that we start to understand it, because increase in ocean acidity could deeply affect marine life along the coast and has important implications for people who rely on aquaculture and fisheries both commercially and recreationally.”

Ocean acidification has been identified as potentially one of the serious impacts of increased concentrations of atmospheric CO2. When the gas is absorbed by the ocean in sets off a chemical reaction that makes the water more acidic. The process disproportionately affects species like oysters, snails, pteropods, and coral, since those organisms cannot effectively form shells in a more acidic environment.

Different regions of coastal ocean will respond to an influx of carbon dioxide in different ways, Wang said.

“If you put the same amount of CO2 in both the Gulf of Maine and the Gulf of Mexico right now, the ecosystem in the Gulf of Maine would probably feel the effects more dramatically,” he said. “Acidity is already relatively high in that region, and the saturation of calcium carbonate — the mineral that many organisms need to make shells—is particularly low. It’s not a great situation.”

Excess CO2 can enter coastal waters from a variety of different sources, Wang said. One large source is carbon dioxide in the atmosphere, which has been steadily increasing in concentration worldwide for the past hundred and fifty years. The higher those levels of atmospheric CO2 rise, more CO2 gas will be absorbed into seawater by contact.

Nutrient-rich runoff from land surfaces also contributes to acidification, Rainfall and other surface flows wash fertilizers and other byproducts of human activities into river systems and ground water, and ultimately, into the coastal ocean, delivering an excess of nutrients and often an explosion of biological activity that can lead to decreased oxygen and increased CO2 and acidity.

“This happens regularly in the Gulf of Mexico,” Wang said. “The Mississippi River dumps enormous amounts of nitrogen and other nutrients into the Gulf, which spawns large algal blooms that lead to production of large amount of organic matter. In the process of decomposing the organic matter, the microbes consume oxygen in the water and leave carbon dioxide behind, making the water more acidic. If this process happens in the Gulf of Maine, the ecosystem there may be even more vulnerable since the Gulf of Maine is a semi-enclosed system and it may take longer time for low pH, low oxygen water to disperse.”

After analyzing their data, Wang and colleagues found that, despite a “dead zone” of low oxygen and high acidity outside the mouth of the Mississippi, the Gulf of Mexico on the whole showed more resistance to acidification. As the team traveled farther north, however, they saw the ratio steadily decreases north of Georgia. The waters in the Gulf of Maine appear especially vulnerable to acidification, Wang said.

For now, the Labrador Coastal Current brings relatively fresh, low alkalinity water down from the Labrador Sea to the Gulf of Maine and Middle Atlantic Bight. But if global warming changes ocean circulation patterns through melting sea ice and glaciers, picture could change.

Since the waters of the northeast U.S. are already susceptible to rising acidity, Wang says this raises big questions about how species of marine life—many of which are important to the commercial fishing and shellfish industry there—will fare in the future.

“For example, how are oysters going to do? What about other shellfish? If the food chain changes, how are fish going to be impacted?” Wang said. “There’s a whole range of ecological and sociological questions.”

 

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