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Blackwater NWR


MidAtlanticLocated on the Eastern Shore of Maryland, the Blackwater National Wildlife Refuge (BNWR) includes over 11,000 hectares of the most extensive tidal wetlands in the Chesapeake Bay. The refuge is a key stopover point for migrating waterfowl along the Atlantic Flyway and is also a haven for several threatened species, including the Delmarva Fox Squirrel. The major wetland habitats in the Refuge are tidal salt marshes, which depend primarily on the accumulation of soil organic matter to keep pace with sea-level rise.

As in many parts of the world, wetlands at BNWR are already vulnerable to the combined effects of land and sea-level change (i.e., relative sea-level rise), and the rate of change in sea level is predicted to accelerate (Church et al. 2001). Emergent vegetation allows the marsh to self-adjust to prevailing water levels through either the accumulation of dead plant shoots on the marsh surface, or, more likely, through plant roots contributing to subsurface soil expansion and maintenance of soil elevation (Cahoon et al. 2004). However, at BNWR vertical marsh development has not kept pace with sea-level rise, and this has led to a loss of more than 3,300 hectares of tidal marsh in the last century (Fig. 2). Current model projections are for continued loss of marsh at BWNR as sea level continues to rise (Fig. 3). Consequently, in recent years identifying the mechanisms behind marsh loss at BNWR has become a focus of management goals at the refuge in an effort to preserve and maintain this valuable ecological resource.

images courtesy of BNWR
Fig. 2. Aerial imagery of Blackwater NWR depicting marsh loss between 1938 (left) and 1989 (right).


Several factors affect the ability of the marshes at BNWR to build vertically through soil matter accumulation and therefore likely influence the rate of ongoing interior marsh breakup. These factors include grazing of vegetation by muskrat and nutria, altered flooding and salinity patterns, annual prescribed burning of vegetation, overabundance of nutrients, subsidence, and changes in the rate of sea-level rise. For example, intense grazing of marsh vegetation by nutria, an exotic species introduced to the United States from South America, severely reduced plant production at BNWR. Following the removal of more than 9,000 nutria from the region between 2002 and 2004, there has been strong recovery of marsh vegetation (M. Haramis, U.S. Geological Survey, written commun., 2007). These findings imply that the combination of sea-level rise and factors affecting sediment accumulation rates will govern the rate of wetland loss along the estuary. Thus, resource managers will have to fully understand the combination of factors affecting marsh loss at a particular site for successful wetland restoration.


Fig. 3. Digital elevation model forecasts of sea-level rise (SLR) at Blackwater NWR in 2002 (top) and 2100 assuming SLR of 6 mm/y (bottom)(modified from Larsen et al. 2004).





Literature Cited
Cahoon, DR, MA Ford, PF Hensel. 2004. Ecogemorphology of Spartina patens-dominated tidal marshes: soil organic matter accumulation, marsh elevation dynamics, and disturbance. Pages 247-266 In: S. Fagherazzi, M Marani, and LK Blum (Eds.) The Ecogeomorphology of Tidal Marshes, Coastal Estuarine Studies volume 59, AGU
Church, J.A., J. M. Gregory, P. Huybrechts, M. Kuhn, K. Lambeck, M. T. Nhuan, D. Qin and P. L. Woodworth, Changes in sea level. in Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, edited by J.T. Houghton et al., p639-693, Cambridge University Press, Cambridge, United Kingdom and New York, New York, USA, 2001.
Larsen, C., I. Clark, G.R. Guntenspergen, D.R. Cahoon, V. Caruso, C. Hupp, and T. Yanosky. 2004. The Blackwater NWR Inundation Model: Rising Sea Level on a Low-lying Coast: Land Use Planning for Wetlands. US Geological Survey, OFR 04-1302, On-line Only, Version 1.0

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