Sea level rise threatens marshes in Chesapeake and Delaware Bays
WASHINGTON - Chesapeake and Delaware Bays, the two largest estuaries on the east coast of the United States, are losing marshland to rising sea levels caused by greenhouse warming. Research by University of Maryland scientists suggests that virtually all coastal marshes along these bays could disappear before 2100, if the sea level continues to rise at present rates or higher rates predicted by climate models.
Loss of these marshes would be devastating, the researchers say, due to its effect on the food chain, water quality, and the amount of carbon that would be released into the oceans and atmosphere. Marshes act as carbon sinks, holding it in solid form, so it does not emerge as carbon dioxide gas. The study, by Prof. Michael S. Kearney and colleagues, is reported in the April 16 issue of Eos, a publication of the American Geophysical Union.
Kearney describes a new technique he and his colleagues developed, based on 1993 images from the Thematic Mapper instrument on the Landsat satellite, updated with more recent aerial photography and field surveys. The model measures reflectance from the marsh's soil or sediment layer, its vegetation, and the water, in order to determine its Marsh Surface Condition Index (MSCI), which tracks the overall health of the marsh. A key benefit of the MSCI is that it helps scientists focus on the role of long term sea level rise, without regard to annual variations caused by heavy storms and other transitory phenomena.
In Chesapeake Bay, the greatest degradation of marshes has occurred in the middle portion of its eastern shore at Blackwater Wildlife Refuge. The upper reaches of both Chesapeake and Delaware Bays are less degraded than the middle and lower reaches. This, the researchers say, is due to the smaller amount of river sediment, which helps the upward growth of the marsh, reaching the lower parts of the bays.
In addition, impoundments, which limit stream flows into Delaware Bay from New Jersey, have resulted in greater degradation of marshes on that shore than on the Delaware shore opposite. From 1984 to 1993, the area of degraded marshes in Delaware Bay increased from 25 percent to 54 percent of total marshland, especially on the New Jersey shore. Currently, in both estuaries, about 70 percent of marshland has been affected, according to the researchers.
Kearney notes that the processes affecting Chesapeake and Delaware Bays could also be at work in other parts of the Atlantic coast. Georgia and South Carolina alone have 300,000 hectares [1,000 square miles] of coastal marshes. Widespread disappearance of these marshes during the 21st century could severely affect coastal ecosystems, particularly food sources at the bottom of the chain. In addition, the sediments now held by the marshes would erode into coastal waters, seriously affecting their quality. Part of this sediment consists of long sequestered carbon, and the potential collapse of hundreds of thousands of hectares [thousands of square miles] of coastal marshes in the coming decades could have a significant effect on the overall North American carbon budget, the researchers say.
The study was supported by grants from the National Aeronautics and Space Administration, the National Oceanic and Atmospheric Administration, Public Service Electric & Gas, and the Environmental Protection Agency.