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Living at a Crossroads - The Restoration of Dyke Marsh Wildlife Preserve

By Dr. Katia Engelhardt, Research Associate Professor, University of Maryland Center for Environmental Science, Appalachian Laboratory.

The significance of tidal freshwater marshes
Tidal freshwater marshes were once extensive along the Coastal Plain rivers of the mid-Atlantic region of the United States.  However, after centuries of intense coastal development, tidal freshwater marshes have been reduced to scattered remnants that can no longer provide the extent of ecosystem services characteristic of widespread, healthy marsh ecosystems. This has far-reaching consequences for the sustainability of wildlife populations and estuarine ecosystems. Nonetheless, even remnant marshes provide numerous benefits and services: resident and migratory wildlife habitat, refuge for endangered species, spawning and nursery grounds for anadromous fish, attenuation of tidal energy, shoreline stabilization, flood control, water quality enhancement, carbon storage, aesthetic enjoyment, and recreational activities.  Consequently, the maintenance and enhancement of the remaining tidal freshwater marshes within the mid-Atlantic regions as well as world-wide are imperative both socially and ecologically.

Tidal freshwater marshes are located at the boundary between tidal and non-tidal riverine wetlands, and they are at risk to be eliminated or severely reduced in extent by sea level rise because there are few suitable stream habitats available for species and marsh migration.  Tidal freshwater marshes have more rare and endangered species than other wetland types and are thus most susceptible to dramatic changes in patterns of species diversity. They differ from other tidal marshes in patterns of nutrient assimilation. Changes to the unique structure and function of tidal freshwater marshes are bound to impact food web dynamics and patterns of nutrient cycling in estuaries.  It is therefore imperative that tidal freshwater marshes are protected and restored to the extent that they can adjust to changes in environmental conditions, such as sea level rise concomitant with other environmental stressors.

The significance of Dyke Marsh Wildlife Preserve
Dyke Marsh Wildlife Preserve is an extremely important tidal marsh, offering invaluable ecosystem services, serving as a fantastic educational classroom, and providing a wide variety of recreational opportunities. Dyke Marsh is one of the largest tidal freshwater marshes located along the Potomac River in the Washington, D.C. area and is viewed as a national treasure because of its proximity to the nation’s capital and a large urban/suburban population, its history, and its current potential for provision of ecological services, recreational values, and educational opportunities.  In addition, the marsh is located along a major travel corridor (G. W. Memorial Parkway) to one of the most popular tourist destinations in the D.C. area (Mount Vernon).

Sedimentation and sea level rise
It is estimated that the formation of Dyke Marsh began 5,000 to 7,000 years ago, although recent coring and dating by the US Geological Survey suggests that the marsh may be significantly younger. Hunting Creek enters the Potomac River immediately upriver of Dyke Marsh.  The velocity of the stream current is decreased as the stream enters the slower moving Potomac River. As a result, deposition occurs.  Twice daily tides carry additional sediments from the Potomac estuary to the area. The constant deposition of sediments ultimately results in marsh surface elevations suitable for marsh vegetation. This deposition is usually sufficient to keep pace with sea level change, where deeper water tends to enhance sediment deposition and shallower water tends to decrease deposition, resulting in the relatively flat marsh platform characteristic of tidal marshes and apparent at Dyke Marsh.

Recent research shows that the rate of marsh accretion (i.e., the rate of mineral and organic accumulation of sediment) is everywhere equal to the rate of sea level rise when sea level rise is steady and moderate. In this case, water depth and biological productivity remain constant through time. An increase in sea level rise, a reduction in sediment supply, and disturbances to marsh vegetation can destabilize the marsh system. In such cases, marshes can erode rapidly and may be irreversibly lost. Projected increases in sea level rise at Dyke Marsh and the region are a) minimal increase of 3 mm/y, the expected increase for the Chesapeake Bay region determined using past records of sea level rise; b) moderate increase in sea level of 6mm/y, which is a median scenario of the Intergovernmental Panel on Climate Change report published in 2001; and c) large increase of 11mm/y, the maximum projected sea level rise according to current scenarios, which may underestimate sea level rise by some accounts. Given these potentially large sea level rise scenarios, it is imperative that disturbances to marsh vegetation are minimized and that sediment supply is maintained, if not enhanced in areas that have seen hydrologic alterations and changes in sediment delivery.

	DMWP in 1937 and 1994 (images at left) highlighting the impacts of dredging, the hardening of terrestrial surfaces and the severe alteration of Hunting Creek (entering the Potomac River at the top left of the images). The image on the right shows the placement of a potential restoration area proposed in a 1977 Environmental Assessment.

Dyke Marsh is currently at an important crossroads. After many years of abuse, continued development of the Potomac River shoreline and watershed, and increased boat traffic on the Potomac River, some marsh shorelines are eroding. Adding the threat of accelerated sea level rise (currently 3mm per year in the area, but could accelerate to 6mm or more), the worry is real whether the marsh will continue to exist in its present state for much longer without human intervention. Still, the current marsh is in excellent health, with vegetation and wildlife communities that are diverse and productive. If Dyke Marsh were a human patient, we might ask whether the patient needs major surgery, a minor procedure, or simply some regular monitoring for vital signs with possible intervention in the future. We might also ask if the patient is even sick or whether we are watching the natural process of aging, which includes changes in form and function.

The case for restoration
A case for restoration can easily be made. Approximately one third of the marsh surface as it existed only 100 years ago was lost to dredging activities. As area is lost, so are species and the capacity to provide ecosystem services. Therefore, an argument can be made from an ethical as well as an ecological perspective that marsh area needs to be restored, at least in part, to regain what was previously lost owing to human negligence.

Dredging left behind deep holes and an altered hydrology. Combined with sea level rise, changes in sediment delivery and increased boat traffic on the Potomac River, sediment deposition and erosion has likely been altered at Dyke Marsh. Thus, even though we still have incomplete information on the rate and extent of marsh surface loss (and potentially gain in other places), one could argue that, at a minimum, hydrology needs to be restored and shorelines stabilized.

The case against restoration
Global warming is a contentious issue. Some people flat out deny global warming is happening; some say it is a natural process so no change in human behavior is necessary; and some argue that humans are accelerating global warming and that a decrease in the carbon foot print is absolutely necessary. Obviously, the same data can be interpreted many different ways. The same arguments can be made for the management and restoration of Dyke Marsh. Not withstanding the loss of marsh area to dredging, it is still unclear whether and how much marsh is concurrently lost and whether this loss is a natural process or accelerated by human activity. Marshes are dynamic systems that constantly lose and gain ground depending on environmental conditions, in particular storm events and sea level. Managing for a static system would be Sisyphean. Hence, one could make the argument to let the marsh be and to allow it to migrate, perhaps, closer to Hunting Creek.

The current marsh is in good health. Any human intervention will increase the chance that the existing marsh will be negatively affected. For example, it is inevitable that new marsh areas will support populations of exotic plants, which then become sources of seeds for the existing marsh. Canada geese may be attracted to new mud flats and then learn to feed in existing portions as well. Engineered structures may add stability to new and existing shorelines but may change the hydrology to the extent that sediment delivery to the existing marsh is altered. A restoration will therefore require careful consideration of alternatives and close collaboration between managers, scientists, and engineers.

Conclusion
Dyke Marsh Wildlife Preserve is an important and irreplaceable ecosystem, and management of the marsh is currently at an important, yet difficult, crossroads. Doing nothing may result in the degradation and migration of the marsh and potential loss of the entire system. Restoration of previously lost areas may greatly enhance the ecosystem, but may also have significant unintended consequences, potentially requiring constant intervention and endless tinkering. A sound, science-based restoration plan is therefore key for minimizing potential adverse impacts.

Editor’s Note: The National Park Service has determined that restoration of Dyke Marsh is feasible and desirable. NPS is currently preparing a wetland restoration and long-term management plan/environmental impact statement which will present several alternatives for public consideration.

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