Project Planning: Salt Marshes
Restoring salt marshes: A regional priority
Restoration vs. creation
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.
Defining project goals and objectives
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).
Baseline data collection
- 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.
Permitting and regulatory considerations
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.
A number of geomorphic, hydrologic, and biotic factors should be considered during the design phase of a salt marsh restoration project. Consideration of nearby seed sources (wind or waterborne) and adequate excavation depths (to eliminate regrowth from buried rhizomes and runners) are critical factors to consider in the elimination of invasive plant species. Care must be taken to ensure that replacement soils do not contain seed banks or rhizome material. Revegetation with desired intertidal vegetation (e.g., Spartina alterniflora) should follow established techniques for propagation and planting, such as those developed by the U.S. Army Corps of Engineer’s (USACE) Dredged Material Research Program (DMRP; Broome et al. 1974; Woodhouse et al. 1974; Seneca et al. 1975, 1976; Barko et al. 1977; Garbisch 1977; Garbisch et al. 1975). Local or regional sources of donor plantings or seedlings are preferable for use in revegetation efforts and are available from commercial nurseries (Broome et al 1974, Seneca et al. 1985).
Tidal channel morphology and the natural dendritic patterns of creeks and channels are important considerations in designing a salt marsh. Tidal creeks and ponds provide “edge” habitat, which is important in maintaining adequate drainage, facilitating nutrient exchange between groundwater and surface waters. Tidal creeks are the pathways for predatory fish to gain access to abundant forage resources within the marsh. Depositional edges of tidal creeks provide access points for smaller finfish and crustaceans, which move on and off the flooded marsh surface to forage and avoid predators (Minello et al. 1994). Channel size and sinuosity can be matched to that observed in reference wetlands. A GIS application can be a powerful tool in identifying appropriate reference areas, and in determining the appropriate amount of edge to be incorporated into a restoration project design.
Potential obstacles to restoration
Phragmites is notoriously persistent and resists many eradication techniques. Tidal flushing, mowing, prescribed burning, and application of chemical herbicides have all been attempted at various locations, often with less than desirable results. Manual cutting, although labor intensive, has been effective in removing Phragmites in conjunction with herbicide application. The herbicide Rodeo, in combination with an organic surfactant, has been used successfully to eradicate Phragmites in southern New Jersey. In many cases, burning invigorates existing stands by removing standing dead biomass. Burning combined with herbicide applications appears to be more successful than prescribed burns alone in controlling Phragmites.
Often the most successful attempts involve multiple control strategies, such as repeated harvesting, with burning to remove accumulated litter. Chemical control typically requires multiple applications over several growing seasons and careful monitoring in order to identify and control reinvasion.
Sometimes, the longer duration of tidal flooding needed for Phragmites control can also result in elimination of desirable salt marsh vegetation. For example, in a restored marsh in Stonington, Connecticut, Phragmites coverage increased following reintroduction of tidal flooding (Sinicrope et al. 1990). This was attributed to the elimination of a freshwater invasive species, Typha. The reduced competition allowed Phragmites to expand aggressively. Ideally, reestablishment of tidal hydrodynamics should be gradual and controlled, in order to avoid subsidence of the peat and permanent flooding.
Many intertidal wetlands contain contaminated sediments. Toxic constituents of note include heavy metals, polychlorinated biphenyls (PCB), polycyclic aromatic hydrocarbons (PAH), and dioxin. Excavation and removal or disposal of these sediments in an environmentally acceptable and economically feasible manner requires careful planning and coordination. Disposal of contaminated sediments can be very expensive, and suitable land- or water-based repositories are scarce. Large-scale sediment decontamination technologies are currently unavailable or so expensive as to be cost-prohibitive. Clean substrates are needed to cap contaminated areas. Sources of clean soil with suitable grain size and organic matter content need to be identified, and an economically feasible means of obtaining such material would need to be determined prior to project implementation.
Animals may destroy new plants
Some animals are known to be highly destructive in their grazing to recently established salt marsh plants. Snow geese, which graze on the soft shoots and rhizomes of smooth cordgrass (Spartina alterniflora), can decimate large areas of newly established marsh within days. On a much smaller spatial scale, muskrat burrowing and feeding activity can also damage newly restored salt marshes. Fences and grids of narrow stakes, which prevent birds from landing in the vicinity of a newly planted or restored area, are often used to prevent snow geese and other animals from disturbing a newly restored site.
Conflicts with surrounding land use
Restoration practitioners must be sure that a project does not conflict with existing land uses such as industrial or commercial facilities in the area, parks and recreational facilities, or existing residential developments. Ideally, these issues are to be resolved in the reconnaissance and planning phase, long before project construction takes place. Hydrologic restoration projects are particularly subject to landowner concerns about increased flooding of adjacent coastal properties (Steinke 1988).
Equipment sources and contacts
Comprehensive literature reviews and technical manuals are available to assist restoration practitioners in project planning. Experts can be contacted at federal and state environmental resource agencies, non-profit organizations, and academic research institutions. The people listed on the Contacts page are available to answer questions.