Increases in atmospheric concentrations of the greenhouse gas carbon dioxide (CO2) may help low-lying coastal marshes keep up with rising sea level by stimulating plant-root production, causing upward expansion of soil surfaces.
According to the Intergovernmental Panel on Climate Change (IPCC), higher CO2 concentrations in the atmosphere are primarily responsible for recent global warming, which contributes to sea-level rise through thermal expansion of oceans and melting of ice fields. To avoid submergence, coastal marshes must match sea-level rise by building vertically—through either surface deposition of mineral sediment or accumulation of organic matter by plants.
Previous work on how coastal wetlands keep up with sea-level rise has emphasized physical processes such as sedimentation and erosion. However, plants and their contribution of organic matter are important to marsh building, especially in sediment-starved areas such as the Mississippi River Delta, according to two recent studies funded by the U.S. Geological Survey (USGS) and published in the Journal of Ecology and the Proceedings of the National Academy of Sciences.
"We knew that biological processes were important in maintaining surface elevations in other peat-forming wetlands, such as mangroves," said Karen McKee, a USGS ecologist. She had previously shown that belowground production of roots by mangroves, a tropical coastal tree, led to a buildup of peat over thousands of years as sea level rose by several meters. "Caribbean islands off the coast of Belize have existed for about 8,000 years and have accumulated over 10 vertical meters [33 ft] of peat—the equivalent of a three-story building," said McKee. More information about this work appears in Global Ecology and Biogeography (v. 16, p. 545-556, URL http://dx.doi.org/10.1111/j.1466-8238.2007.00317.x)
The recent studies examined processes controlling elevations in coastal marshes in the Mississippi River Delta and the Chesapeake Bay—two areas particularly vulnerable to sea-level rise. The scientists hypothesized that rising levels of atmospheric CO2, which is known to stimulate plant growth, might boost the buildup of marsh elevation, helping to counterbalance sea-level rise.
Both studies were based in part on research conducted in the Wetland Elevated CO2 Experimental Facility, a USGS facility at the National Wetlands Research Center (NWRC) in Lafayette, Louisiana. The facility is designed to investigate the impacts of CO2 and other climate-change factors on wetlands. The marsh studies are part of a larger project funded by the USGS Global Change Research Program.
McKee collaborated with Jim Grace (NWRC) and Julia Cherry (former USGS postdoctoral scientist who is currently an assistant professor at the University of Alabama) to examine the responses of a brackish-marsh community to combinations of CO2, flooding, and salinity by using mesocosms—miniature marshes made of segments of marsh from the Mississippi River Delta and established in the greenhouses of the Wetland Elevated CO2 Experimental Facility. The investigators varied the CO2 concentration in each greenhouse; within each mesocosm, they varied salinity and simulated various levels of flooding.
"We found that higher concentrations of CO2 ameliorated salt stress for one of the plant species in this brackish-marsh community, and better plant growth drove faster soil expansion in mesocosms," said McKee.
The data from this experiment were used to develop a model to better understand the biophysical mechanisms controlling vertical marsh expansion. "Our work shows that it’s important to include biological feedback mechanisms in models used to predict inundation of coastal areas under sea-level rise scenarios," said Grace, whose expertise is in modeling and analysis of complex systems. Their results were reported in the January 2009 issue of the Journal of Ecology (v. 97, no. 1, p. 67-77, URL http://dx.doi.org/10.1111/j.1365-2745.2008.01449.x)
The second study, funded in part by the USGS, was led by the Smithsonian Environmental Research Center (SERC). McKee and Cherry worked with Adam Langley (SERC), Pat Megonigal (SERC), and Don Cahoon (USGS) to examine elevation response to CO2 in a Chesapeake Bay marsh. The authors observed that higher CO2 concentrations increased the rate of marsh elevation gain, primarily by stimulating plant-root production.
"By combining the results of this field study with findings from the NWRC mesocosm experiment, we were able to show that the CO2 effect observed under current sea-level conditions in the Chesapeake Bay marsh would vary depending on salinity and flooding conditions associated with future sea-level rise," said McKee. The Chesapeake Bay study was reported in an article titled "Elevated CO2 Stimulates Marsh Elevation Gain, Counterbalancing Sea-Level Rise," published in the April 14, 2009, issue of Proceedings of the National Academy of Sciences (v. 106, no. 15, p. 6182-6186, URL http://dx.doi.org/10.1073/pnas.0807695106).
Wetland loss is a severe problem along parts of the U.S. coast. In some areas of coastal Louisiana, for example, wetland-building processes no longer keep pace with wetland loss, owing to a combination of factors such as sea-level rise, ground subsidence, and dams and levees that prevent coastal wetlands from receiving the river water, nutrients, and sediment needed to nourish wetland vegetation. (For example, see "Slowing of Coastal Subsidence Is Good News for Restoration of Louisiana's Wetlands," Sound Waves, October 2008). The insights gained from the NWRC studies described here, as well as other investigations conducted by the USGS, will contribute to a better understanding of how wetlands may respond to rising sea level and related factors. Such research will be essential to accurately predict how global climate change will impact coastal areas in the future.
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