Ocean acidification sampled waters off the US east coast aboard the R/V Ron Brown. (Map: Z. Aleck Wang)
Broad ocean acidification survey breeds clues
Tuesday, March 05, 2013, 23:10 (GMT + 9)
A continental-scale chemical survey in the waters of the eastern US and Gulf of Mexico is helping researchers determine how distinct bodies of water will resist changes in acidity. The study, which measures varying levels of carbon dioxide (CO2) and other forms of carbon in the ocean, was conducted by scientists from 11 institutions across the US 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 US," says Zhaohui 'Aleck' Wang, the study's lead author and a chemical oceanographer at Woods Hole Oceanographic Institution (WHOI). "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."
According to the survey, says Wang, different regions of coastal ocean will respond to an influx of CO2 in different ways.
"If you put the same amount of CO2 into 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 says. "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."
|Wang, far right, and his students and lab members (clockwise from far left, Elliott Roberts, Kelly Knorr, Katherine Hoering, Sophie Chu, and Nick Tuttle) at sea around a CTD rosette. CTDs measure conductivity or salinity, temperature, and depth and are a common tool of oceanographic research. Water samples are collected in each of the rosette’s canisters at various depths and then chemically analyzed in the lab.
(Photo Credit: Taylor Crockford, Woods Hole Oceanographic Institution)
During the research cruise, the researchers measured seawater samples for total dissolved inorganic carbon (DIC), which is made up of a combination of carbonate, bicarbonate, dissolved CO2 and carbonic acid. The team compared this measurement to the water's total alkalinity, a measure of how much base is in a water sample.
The ratio of the two is a marker for water's ability to "buffer" or resist changes in acidity. Waters with a high ratio of alkalinity to DIC, Wang says, would be less susceptible to acidification than waters that showed a much lower ratio.
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 a high ratio of alkalinity to DIC, meaning it would be more resistant to acidification. As the team travelled farther north, however, they saw the ratio steadily decreases north of Georgia.
The waters in the Gulf of Maine, Wang says, on average had the lowest alkalinity to DIC ratio of any region along the eastern seaboard, meaning that it would be especially vulnerable to acidification should CO2 levels rise in those waters.
While it's unclear exactly why the ratio of alkalinity to DIC is low in those northern waters, Wang thinks part of the issue may be linked to alkalinity sources to the region. For example, the Labrador Coastal Current brings relatively fresh, low alkalinity water down from the Labrador Sea to the Gulf of Maine and Middle Atlantic Bight.
If this current is the major source of alkalinity to the region, he says, it may mean that the Gulf of Maine's fate could be linked to changes in global climate that, through melting sea ice and glaciers, increase the flow of fresh water to the Gulf of Maine. However, whether this freshening is accompanied by a decrease in seawater alkalinity and "buffer" capacity remains unknown.
There is a great need for more robust coastal ocean chemistry monitoring and coastal ocean acidification studies, Wang adds. A better understanding of the changing chemistry will help fisheries regulators to better manage the stocks.
- Grants for ocean acidification research on commercial fisheries