Project Planning: Salt Marshes

A critical factor in the selection of restoration sites is analysis of current and projected land-use patterns and socioeconomic factors. Cooperation from landowners and municipalities and the cultivation of favorable public perception of habitat restoration are needed to achieve success on a landscape scale (USACE-NED 2002).

Many potential sites for salt marsh restoration exist along the Gulf of Maine’s coastline. Evaluation of site-specific factors (e.g., presence of invasive plant species, history of dredged material or other fill placement, hydrologic restriction, ownership, adjacent land use) can be used to prioritize potential restoration sites.

Project objectives are more precise. They focus on specific characteristics of water quality, hydrology, or plant and animal communities to be restored. Performance indicators are developed for measurable characteristics, such as nitrogen concentration in soil pore waters, Phragmites height and stem density, or number of killifish per unit area of marsh surface. These quantitative indicators are used to evaluate whether a salt marsh restoration project will meet or has met predetermined success criteria toward the project goals. For example, if the goal is to reduce or eliminate Phragmites, the success criteria might specify the target number of acres recolonized by Spartina species at 3, 5, and 10 years post-restoration. Failure to meet these acreage targets would be cause for reevaluation of the project and perhaps implementation of corrective measures (Pastorok et al. 1997, Weinstein et al. 1997).

Environmental factors to be considered in the formulation of project goals and objectives include hydrologic conditions, surrounding land-use patterns, connectivity to natural wetlands or other adjacent ecosystems, and evaluation of various wetland functions. Enhancement of certain functions (e.g., habitat for estuarine-dependent fisheries) may be a primary objective of many salt marsh restoration projects. However, designing a project to maximize the output of one particular function may involve a trade-off with other functions (e.g., bird and wildlife habitat or shoreline stabilization).

• surface topography and elevation,
• water table depth,
• surface water level,
• surface and groundwater quality,
• soil organic matter and water content,
• plant species distribution and cover,
• benthic invertebrate communities,
• use of the marsh by finfish and crustaceans, and
• use of the marsh by wildlife.

For more information, consult the Monitoring page.

Reestablishment of tidal hydrodynamics is a critical first step in the restoration process. Tidal restriction due to dikes, levees, and poorly designed water-control structures leads to a substantial reduction in pore-water salinity, lowering of the water table, and a relative drop in marsh surface elevation (Roman et al. 1984, Rozsa 1988). All of these conditions favor the establishment of Phragmites. Spartina marshes that historically have been subjected to extensive diking and ditching have experienced rapid and widespread Phragmites invasion.

Hydrologic restoration of salt marshes in the region began in the late 1970s. Breaching of existing dikes, and modifications to tide gates and other water control structures in order to recreate historic tidal flushing regimes has resulted in the reestablishment of native salt marsh vegetation at many restoration sites along the coast. Reintroduction of tidal flooding can result in a substantial decrease in Phragmites height and vigor within one growing season. Vegetation change is most apparent in lower intertidal areas and along creeks and ditches, where tidal inundation is greatest. However, restoration of an entire marsh is gradual, and it may take decades following reestablishment of tidal flooding (Sinicrope et al. 1990; Roman et al. 1984, 1995; Rozsa 1988).

In other areas, where the degree of tidal flooding is sufficient, or where removal of water control structures or dikes is not feasible, restoration may focus primarily on removal of Phragmites and replanting with native intertidal vegetation.

Removal of dredged material from marsh surface
Many salt marshes have been impacted by the deposition of fill, either mud and sand dredged to aid navigation or structural fill derived from industrial activities. Filling salt marshes has persisted for centuries. Originally, it was considered beneficial because it converted marsh “wasteland” to usable “fast-land.” In the last few decades, however, people have recognized the benefits and functions of intact salt marshes, and the United States has enacted legislation to protect marshes from filling.

To restore filled marshes, the historic intertidal elevations must be reestablished with earth-moving equipment. Elevation criteria for the recontouring effort can be obtained from surveys of nearby reference marshes in similar geomorphic and landscape settings. Proper elevation is critical to achieve the tidal flooding characteristics for sustaining the desired plant community (e.g., Spartina alterniflora-dominated low marsh). After dredged material or upland fill is removed, suitable soils can be placed on the site. The soil’s organic-matter content and grain size should match that of reference marshes. If the organic-matter content is too low, restoration practitioners can add organic matter (usually terrestrial vegetation mulch) to enhance soil quality. However, this additional step can be expensive and time-consuming. Fortunately, Spartina spp. is well-adapted to sandy, low-nutrient soils, and it is relatively easy to propagate on properly prepared restoration sites (Broome et al. 1974; Woodhouse et al. 1974; Seneca et al. 1975, 1976; Barko et al. 1977; Garbisch 1977; Garbisch et al. 1975).

Invasive species removal
Ecologists are only now beginning to understand the ecological consequences of Phragmites expansion in tidal marshes. Accumulation of inorganic sediments and organic biomass associated with the rapid growth of Phragmites increases marsh elevation and decreases tidal flooding (Windham and Lathrop 1999, Rooth and Stevenson 2000). Reduced tidal exchange limits production of fishery species that use marsh surface and tidal creek habitats as a predation refuge, forage site, and spawning area (Weinstein and Balleto 1999). Benthic invertebrates may be affected negatively by reduction in flooding depth and duration, and decreased litter quality (Able and Hagan 2000, Angradi et al. 2001). Phragmites marshes are generally considered to be low-quality foraging and nesting habitat for birds and wildlife. However, some species (e.g., red-winged blackbird) are known to use Phragmites extensively as nesting habitat (Benoit and Askins 1999). Restoration of intertidal wetlands through eradication of Phragmites and revegetation with native, non-invasive plant species should net an overall improvement in habitat quality for fishery and wildlife species, and maintain marsh-open water trophic linkages.

In addition to maintaining natural ecological processes and trophic dynamics, removal of Phragmites helps to enhance biodiversity in the coastal zone. Elimination of the vast, monotypic stands of Phragmites can lead to an increase in plant species diversity. Maintenance of characteristic invertebrate, fish, and wildlife communities will also contribute to improvement of and maintenance of biodiversity in coastal habitats.

In this region, most projects intended to eliminate Phragmites have pursued that goal by increasing tidal flooding. The gradual accumulation of sulfides (an important component of seawater) in flooded marsh soils inhibits the ability of Phragmites to take up nutrients. Eventually, Phragmites stands lose vigor and height, and die back (Chambers 1997). This process can take up to several years. It is also possible to eradicate Phragmites using herbicides, burning, and manual harvesting. These techniques have been used extensively in other areas (e.g., New Jersey), where restoration of tidal hydrology is not possible. They have also been used in combination with restored tidal flooding in order to accelerate the process of habitat restoration. These techniques are expensive and difficult to implement. In the case of herbicides, multiple applications over several growing seasons may be necessary to maintain a Phragmites-free plant community. Harvesting is labor intensive, and the cuttings must be disposed in a way that prevents spreading of seeds and rhizomes to other locations. Burning of Phragmites has been conducted in a few instances, but the approach is not advised in marshes adjacent to residential areas.