Scientific findings of the U.S. Geological Survey's National Wetlands Research Center in Lafayette, La., were presented at a seminar sponsored by the U.S. Global Change Research Program Thursday in Washington, D.C.
Dr. Virginia Burkett, branch chief of forest ecology at the center, discussed the center's research on the relationship of global climate change to coastal wetlands at a seminar on "Wetland Loss in the United States: Scope, Causes, Impacts, and Future Prospects."
She followed introductions by Dr. Rosina Bierbaum, Assistant Director for the Environment, White House Office of Science and Technology Policy, and a presentation by Dr. Mark Schaefer, Deputy Assistant Secretary for Water and Science, U.S. Department of the Interior, Washington, D.C. Schaefer spoke on the importance of U.S. wetlands and causes of wetland loss, including the latest trend statistics and graphics from the department's National Wetlands Inventory.
Effects of Possible Global Climate Change
Although some uncertainty and disagreement exist in the scientific community about magnitude of change, the potential impacts on wetlands are of great practical concern to those living and working in coastal areas, according to Dr. Robert E. Stewart, Jr., center director."Among the coastal areas of greatest risk in the United States are those low-lying coastal habitats that are easily eroded and occur along the northern Gulf of Mexico and southern Atlantic coasts of the United States. These coastal wetlands are especially vulnerable to direct, large-scale effects of climate change, primarily because of their sensitivity to sea-level rise," he said.
Global sea level has already risen between 10 and 25 centimeters (about 4 to 10 inches) over the past 100 years. The Intergovernmental Panel on Climate Change in 1995 projected for the next century a sea-level rise of 15-95 centimeters (about 6 to 37 inches) due to thermal expansion and melting of glaciers and ice sheets. Continued sea-level rise will increase the depth of coastal waters and increase inland and upstream salinity intrusion, both of which affect brackish and freshwater wetlands.
While there is uncertainty about the influence of global warming on the frequency and intensity of storms, sea-level rise alone has the potential for increasing the severity of storm surges, particularly in areas where coastal habitats and barrier shorelines are rapidly deteriorating, Dr. Burkett said. These direct consequences of global- and regional-scale changes will increase the vulnerability of coastal wetlands (including mangrove and salt marshes) that are already heavily affected by human activities, she added.
Research at the USGS National Wetlands Research Center
Dr. Burkett gave an overview of the center's coastal research on global change, covering such issues as the impact of sea- level rise, storms, and increased salinity and atmospheric carbon dioxide on seagrasses, coastal marshes, mangrove forests, baldcypress swamps, and bottomland hardwood forests. Center scientists work with various other federal and state agencies as well as universities to study global change.Center scientists have helped forecast possible changes to these wetlands by modeling the potential for increased coastal flooding and increased storm damage related to sea- level rise. Remote sensing studies have also aided global change research by developing radar and optical sensing tools capable of detecting and monitoring coastal flooding and changes in hydrology, soil moisture, and depth to groundwater table.
Center study results show that changes in sea level are expected to alter the plant community composition and aerial coverage of seagrass beds. Field studies off the coasts of Mississippi and Louisiana have shown, for example, that barrier island overwash is a major determinant of seagrass community composition in the Chandeleur Sound, which contains some of the largest grassbeds in the north central Gulf region.
Because different species of seagrass support different organisms, changes in the composition of seagrasses will be felt at every level of the food web, including shellfish, finfish, waterfowl (especially redhead ducks, which depend almost entirely on seagrass for their winter diet), and people.
Farther inland, salinity intrusion becomes more important than disturbance in controlling the species in submerged grassbeds. Some species have a higher tolerance to salt water and will quickly outcompete those that are salt-intolerant. Other species may be completely eliminated, or may possibly be displaced inland, depending on the rate at which sea level rises. Most of the U.S. southeastern coastal zone is highly developed, however, and the inland migration of seagrasses and salt marshes will be prevented in many areas by existing structures such as seawalls, roads, and other construction.
Center research at sites in five coastal states indicates that many marshes and mangrove ecosystems receive adequate mineral sediments to produce enough organic sediment and root material to remain above sea level at the present rate of sea-level rise (1-2 mm or a small fraction of an inch per year globally). Three of the 12 wetlands studied, however, were not keeping pace with the current rate of sea-level rise. If sea-level rise accelerates, some additional sites would also begin to slowly deteriorate and submerge. In some areas, the sinking or subsidence of the marsh system, coupled with human development, is the major cause of wetland loss.
In Louisiana, Florida, and South Carolina, submergence and saltwater intrusion have been implicated in the decline in productivity and death of certain coastal forests. Restoration of salinity-affected baldcypress swamps, however, may be possible by cultivating more salt-tolerant strains of baldcypress that the center has discovered.
Studies in Texas have also shown that changes in global climate would affect bottomland hardwood forests and other forests of the coastal plain by possibly influencing patterns of disturbances such as fires or storms and by altering the regional moisture balance. Such changes would affect both preserved and commercial forests of the coastal plain.
Computer model simulations of the combined influence of sea- level rise, storms, and floods suggest that large areas in the southeastern United States could be converted from coastal marsh to open water and from forest to marsh. Hurricane model simulations also suggest that if hurricanes or the effects of hurricanes become more intense over the next century, wetland community structure and composition would be altered. For example, historical simulations suggest that the occurrence of major hurricanes is the most important factor controlling mangrove community dynamics.
Given the likelihood of environmental responses to global change, USGS scientists continually strive to improve monitoring techniques. Remote sensing research, Burkett says, has yielded tools capable of monitoring coastal conditions regardless of weather and darkness.
Fact Sheets Available
The center has produced eight fact sheets discussing the details of these studies. They are available from the
National Wetlands Research Center Library
700 Cajundome Blvd.
Lafayette, La., 70506
or can be ordered from 318-266- 8692. Information is also available on the U.S. Global Change Research Program's web site at http://www.usgcrp.gov/usgcrp/9777DD.html.