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Sea sickness

Report warns about carbon dioxide threats to marine life


by David Hosansky


Coral reef organisms and the reefs they build are both affected by ocean acidification. (Photo courtesy of Florida Keys National Marine Sanctuary.)

Worldwide emissions of carbon dioxide from fossil fuel burning are dramatically altering ocean chemistry and threatening marine organisms, including corals, that secrete skeletal structures and support oceanic biodiversity. A landmark report released in July summarizes the known effects of increased atmospheric carbon dioxide on these organisms, known as marine calcifiers, and recommends future research.

"It is clear that seawater chemistry will change in coming decades and centuries in ways that will dramatically alter marine life," says NCAR scientist Joan Kleypas, the report's lead author. "But we are only beginning to understand the complex interactions between large-scale chemistry changes and marine ecology. It is vital to develop research strategies to better understand the long-term vulnerabilities of sensitive marine organisms to these changes."

The report, "Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers," warns that oceans worldwide absorbed approximately 11billion metric tons of carbon between 1800 and 1994. Oceans are naturally alkaline, and they are expected to remain so (see sidebar), but the interaction with carbon dioxide is making them less alkaline and more acidic. The increased acidity lowers the concentration of carbonate ion, a building block of the calcium carbonate that many marine organisms use to grow their skeletons and create coral reef structures.

"This is leading to the most dramatic changes in marine chemistry in at least the past 650,000 years," says Richard Feely, one of the authors and an oceanographer at NOAA's Pacific Marine Environmental Laboratory (PMEL) in Seattle.

The report follows a workshop funded by NSF and NOAA and hosted by the U.S. Geological Survey (USGS) Integrated Science Center in St. Petersburg, Florida.

Experimental studies, such as those conducted by one of the report's authors, Chris Langdon (University of Miami), show that coral calcification consistently decreases as the oceans become more acidic. This means that these organisms will grow more slowly, or their skeletons will become less dense, a process similar to osteoporosis in humans. As a result, reef structures are threatened because corals may be unable to build reefs as fast as erosion wears them away.

Joan Kleypas

Joan Kleypas, one of the authors of the NSF/NOAA/USGS report. (Photo by Carlye Calvin.)

"This threat is hitting coral reefs at the same time that they are being hit by warming-induced mass bleaching events," Langdon says. Mass bleaching occurs when unusually warm temperatures cause the coral to expel the colorful microscopic algae that provide the coral polyps with food.

More than reefs at risk

Many calcifying organisms—including marine plankton such as pteropods, a planktonic marine snail—are affected by the chemistry changes. Shelled pteropods are an important food source for salmon, mackerel, herring, and cod. If calcifying organisms such as pteropods are unable to sustain their populations, many other species may be affected.

"Decreased calcification in marine algae and animals is likely to impact marine food webs and has the potential to substantially alter the biodiversity and productivity of the ocean," says Victoria Fabry (California State University, San Marcos), another of the report's authors.

Several other major ecosystems that are supported by marine calcifiers may be particularly threatened by ocean acidification. These include cold-water reefs, which are extensive structures that provide habitat for many important fish species, particularly in the coastal waters of Alaska.

The report outlines future research to understand this consequence of climate change. While scientists cannot yet fully predict how much marine calcification rates will change in the future, the report warns that the more critical question is: "What does this mean in terms of organism fitness and the future of marine ecosystems?"

Lisa Robbins (USGS Center for Coastal and Watershed Studies) and Chris Sabine (PMEL) also co-authored the report, which is designed to serve as a guide to program managers and a resource for researchers, including graduate students, who are interested in conducting work on marine carbonate chemistry and calcification.

The basics of ocean acidification


The phrase "ocean acidification" can be confusing, because the oceans are actually slightly alkaline and expected to remain so even as greenhouse gases increase. In the NSF/NOAA/USGS report discussed above, the authors explain that they use the phrase "to conform with current terminology, with the recognition that it refers to the process rather than an end state."

On the logarithmic pH scale used to measure acidity, seawater near the ocean surface averages about 8.2, according to a 2005 report by the Royal Society (Great Britain). Local and seasonal variations can push that number up or down by about 0.3. Pure water—neither acidic nor basic—has a pH close to 7.0. By comparison, lemon juice has a pH of around 2.4; coffee, about 5.0; and laundry bleach, around 12.5.

Overall, greenhouse gases may have pushed the typical pH value of seawater down by about 0.1 unit since preindustrial times, according to a 2003 modeling study in Nature by Ken Caldeira and Michael Wickett (Lawrence Livermore National Laboratory). The decrease of 0.1 unit equates to a 30% increase in the concentration of hydrogen ions—or, as the 2005 Royal Society report puts it, "a considerable acidification of the oceans."

Using a midrange scenario of greenhouse emissions, as calculated by the Intergovernmental Panel on Climate Change, the NSF/NOAA/USGS study estimates that near-surface oceanic pH could drop by 0.4 unit by the year 2100. Although this would leave the oceans still slightly alkaline, it corresponds to a threefold increase in hydrogen ion concentration since preindustrial times. According to the Royal Society report, it may take tens of thousands of years before pH values return to preindustrial levels.


On the Web

NSF/NOAA/USGS report (PDF, 9.7mb)

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