Acid Ocean

Burke Hales, OSU College of Oceanic and Atmospheric Sciences (Photo: Don Frank)
Burke Hales, OSU College of Oceanic and Atmospheric Sciences (Photo: Don Frank)

Water that upwells seasonally along the West Coast of North America is growing increasingly acidic, according to a survey conducted in 2007 by an international team of scientists. In June, they reported finding acidified ocean water within 20 miles of the shoreline, raising concern for marine ecosystems from Canada to Mexico.

Deep-ocean currents take years to transport acidified water to upwelling regions, say members of the research team, which included Burke Hales, an associate professor in the College of Oceanic and Atmospheric Sciences at Oregon State University. Thus it is likely that increasingly acidic water will continue to upwell along the West Coast in the future, they add.

“The coastal ocean acidification train has left the station, and there not much we can do to derail it,” says Hales, an author of a report published in Science. The research was funded by the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA).

In their survey, the researchers used the Wecoma, an OSU research vessel owned by the National Science Foundation, to collect water samples at pre-determined points off shore. They found indications that acidified water in upwelling regions had previously been at the ocean surface about 50 years ago. At that time, atmospheric CO2 levels were roughly 310 parts per million.

Since then, CO2 levels have risen in the atmosphere by about 20 percent. When it reacts with water, CO2 generates carbonic acid, which, at high enough concentrations, can harm shell-building organisms such as corals, clams, snails and oysters. Scientists call such water “corrosive” because it can weaken shells and coral reefs.

The study was the first in a planned series of biennial observations of the carbon cycle along the West Coast. In addition to Hales, principal investigators included Richard A. Feely and Christopher Sabine of the NOAA Pacific Marine Environmental Laboratory; J. Martin Hernandez-Ayon, the University of Baja California in Mexico; and Debby Ianson, of Fisheries and Oceans Canada, Sidney, B.C.

Shells at Risk

Pink scallop (Illustration courtesy of Oregon Sea Grant)
Pink scallop (Illustration courtesy of Oregon Sea Grant)

“When the upwelled water was last at the surface, it was exposed to an atmosphere with much lower CO2 levels than today’s,” Hales points out. “The water that will upwell off the coast in future years already is making its undersea trek toward us, with ever-increasing levels of carbon dioxide and acidity.”

Scientists have become increasingly concerned about ocean acidification in recent years, as the world’s oceans absorb growing levels of carbon dioxide from the atmosphere. Carbonic acid has a corrosive effect on aragonite, the calcium carbonate mineral that forms the shells of many marine creatures.

Certain species of phytoplankton and zooplankton, which are critical to the marine food web, may also be susceptible, the scientists point out, although other species of open-ocean phytoplankton have calcite shells that are not as sensitive.

“There is much research that needs to be done about the biological implications of ocean acidification,” Hales says. “We now have a fairly good idea of how the chemistry works.”

Atmospheric CO2 levels form the beginning baseline for carbon levels in ocean water. As water sinks away from the surface and moves toward upwelling areas, CO2 levels also rise from the normal process of respiration by plants and animals. As that nutrient-rich water is upwelled, it triggers additional phytoplankton blooms that continue the process.

Dead Zones and Acidification

There is a strong correlation between recent hypoxia events off the Northwest coast and increasing acidification, Hales says.

“The hypoxia is caused by persistent upwelling that produces an over-abundance of phytoplankton. When the system works, the upwelling winds subside for a day or two every couple of weeks in what we call a ‘relaxation event’ that allows that buildup of decomposing organic matter to be washed out to the deep ocean.

Razor clam (Illustration courtesy of Oregon Sea Grant)
Razor clam (Illustration courtesy of Oregon Sea Grant)

“But in recent years, especially in 2002 and 2006, there were few if any of these relaxation breaks in the upwelling, and the phytoplankton blooms were enormous,” Hales adds. “This decomposition puts more CO2 into the system and increases the acidification.”

The researchers found that the 50-year-old upwelled water had CO2 levels of 900 to 1,000 parts per million, making it “right on the edge of solubility” for calcium carbonate-shelled aragonites, Hales says.

“If we’re right on the edge now based on a starting point of 310 parts per million, we may have to assume that CO2 levels will gradually increase through the next half century as the water that originally was exposed to increasing levels of atmospheric carbon dioxide is cycled through the system. Whether those elevated levels of carbon dioxide tip the scale for aragonites remains to be seen.

“But if we somehow got our atmospheric CO2 level to immediately quit increasing,” Hales adds, “we’d still have increasingly acidified ocean water to contend with over the next 50 years.”

Variation is the Rule

Mussel (Illustration courtesy of Oregon Sea Grant)
Mussel (Illustration courtesy of Oregon Sea Grant)

Hales says it is too early to predict the biological response to increasing ocean acidification. There is already a huge seasonal variation in ocean acidity based on phytoplankton blooms, upwelling patterns, water movement and natural terrain. Upwelled water can be pushed all the way onto shore, he says, and barnacles, clams and other aragonites have likely already been exposed to corrosive waters for a period of time.

They may be adapting, or they may already be suffering consequences that scientists have not yet determined.

“You can’t just splash some acid on a clamshell and replicate the range of conditions the Pacific Ocean presents,” Hales says. “This points out the need for cross-disciplinary research. Luckily, we have a fantastic laboratory right off the central Oregon coast that will allow us to look at the implications of ocean acidification.”

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