Managing marsh loss using sediment enhancement to preserve habitat for waterfowl

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This article is part of the Spring 2016 issue of the Earth Science Matters Newsletter

Tidal marshes are highly productive wetlands characterized by fine-grained sediments, low or no wave action, and frequent tidal flooding. They are found predominantly in sheltered intertidal areas of the mid- and high-latitudes of the world. Tidal marshes are increasingly vulnerable to wetland loss attributable to rising sea level due to global temperature increases. The Blackwater River, a large brackish estuary on the Eastern Shore of the Chesapeake Bay, is an example of a wetland that has seen drastic rates of marsh loss in the last century (Figure 1).

Brackish tidal marsh

Figure 1: Brackish tidal marsh at Blackwater National Wildlife Refuge, MD.

(Credit: James Lynch, USGS. Public domain.)

Elevation of Blackwater National Wildlife Refuge

Figure 2: LIDAR image of the Blackwater NWR highlighting low elevation marsh habitat susceptible to degredation and conversion to open water (March 25, 2002; contour interval 30 cm; datum NAVD 88; after Larsen et al. 2004, USGS Open File Report 04-1302).

(Public domain.)

Blackwater National Wildlife Refuge (Blackwater NWR) was originally established in 1933 on the Blackwater River near Cambridge, MD as a sanctuary for migratory birds traveling along the Atlantic Flyway (Figure 2). Blackwater NWR contains over 11,700 hectares (ha) of diverse habitat suitable for wildlife, a large portion of which is brackish tidal marsh dominated by saltmeadow cordgrass (Spartina patens) and Olney’s three-square (Schoenoplectus americanus). These Refuge lands provide critical food resources and shelter for migrating waterfowl including Canada geese, snow geese, tundra swan, and more than 20 species of duck. Since Blackwater NWR was established, more than 2,000 ha of marsh have been lost due to sea-level rise and subsidence (the gradual sinking of land). Maximizing suitable marsh habitat is crucial to the waterfowl management strategies for the Refuge; a loss of this habitat means a loss of the goods and services available for migratory and resident wildlife species.

Rod surface elevation table

Figure 3: Rod surface elevation table at Blackwater National Wildlife Refuge, MD.

(Credit: James Lynch, USGS. Public domain.)

For nearly 20 years, researchers from the U. S. Geological Survey Climate Research & Development Program (USGS) have partnered with the U. S. Fish & Wildlife Service (USFWS) Blackwater NWR to study the biological and physical processes driving the loss of their marsh. USGS researchers first developed an ecological model that predicted conversion and loss of refuge habitat with varying rates of sea-level rise. The model allowed Blackwater NWR natural resource managers to identify areas of the refuge most vulnerable to rising sea levels and forecast degradation of marsh habitat and eventual loss to open water.

USGS researchers then conducted a series of experiments on above- and below-ground plant growth, decomposition, and sedimentation to better understand the processes influencing the observed changes on the refuge. They employed surface elevation tables which make precise measurements of the soil surface to study small scale changes in the elevation of the marsh (Figure 3). The USGS also conducted experiments using “marsh organs” to determine the flood tolerance of S. patens and S. americanus (Figure 4). Marsh organs are PVC planters constructed at varying heights within the tidal cycle. They are designed to measure the impact of rising water levels and determine the optimum soil surface elevation and flooding height for maximum growth and development of target plant species. These studies suggested that most of the areas in the Blackwater NWR covered by marsh vegetation were below the optimum elevation necessary to maintain plant health and persistence given current rates of sea-level rise.

"Marsh Organ" planted with Spartina species

Figure 4: Marsh organ planted with Spartina species.

(Credit: Gregg A. Snedden, USGS. Public domain.)

After many years of rigorous research and collaboration, USGS scientists and Blackwater NWR staff identified a target elevation that will support the growth and development of S. patens and S. americanus. In 2016, the refuge plans to artificially add sediment to raise the level of the marsh surface to an elevation above the critical minimum sufficient for the production of these requisite plant species. This new management strategy should maximize plant belowground production, mitigate sea-level rise vulnerability, and enhance overall wetland stability.

According to the Blackwater NWR staff, USGS data have informed and reformed their management decisions greatly. Prior to this research effort, the goals and visions of the Blackwater NWR Comprehensive Conservation Plan were based on the best science available, but without a clear understanding of the mechanisms influencing habitat conversion on the refuge. The long-term partnership between the USGS and the USFWS at the Blackwater National Wildlife Refuge has resulted in the production of essential datasets describing the processes contributing to wetland loss and the development of management strategies to slow or reverse these trends and enhance favorable waterfowl habitat.

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