The Health and Long Term Stability of Natural and Restored Marshes in Chesapeake Bay

Abstract

Recent evidence in Chesapeake Bay suggests that the majority of the tidal marsh acreage has been negatively affected by sea-level rise in recent years. The ultimate impact, total marsh loss, centers on Blackwater National Wildlife Refuge (NWR) which has encountered particularly high rates of land subsidence in the twentieth century. Analysis using digitized photography has revealed that marshes are being lost more rapidly in the northern than southern sections of Blackwater. The former is closest to the center of a large cone of depression in the most important underlying aquifer in the region. The groundwater withdrawals at Cambridge correspond to a rapid rise in sea level which appears to be two to three times the present global rate of 1 to 2 mm y−1. The declining health of the marshes, reflected in reduced productivity, canopy thinning, channel enlargement, rotten spots and salt pans as well as ultimate conversion to mudflat and open water, can be tracked using satellite imagery. Although not as dramatic as marsh conversion to open water at Blackwater NWR (over 50% during the 20th century), other marshes on the lower Eastern Shore of Maryland show clear signs of incipient change. Past attempts at restoring Blackwater marshes have not been successful due in part to the combination of excessive grazing by muskrats and nutria as well as anthropogenic influences (reduced diel tidal amplitude because of road building, increasing salinity because of canals, and possibly large-scale burning).Ultimately, restoration efforts depend on the maintenance of groundwater pressure and/or on supplementation of the system with sediment from other sources to keep them abreast of rising sea level. Restoration efforts will depend not only on controlling groundwater withdrawals, but possibly in revitalizing existing marshes by promoting rhizosphere oxygenation. Where these strategies are not practical, more innovative approaches may be necessary such as the use of highly productive species (e.g., Phragmites australis) that appear to be more efficient in promoting sedimentation and long term accretion than other marsh species.KeywordsLand SubsidenceTidal MarshPhragmites AustralisMarsh SurfaceGroundwater WithdrawalThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.