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Tidal marsh restoration is the practice of returning a degraded tidal marsh to a condition that more closely resembles that of an undisturbed site. The aim of a restoration project is usually to develop a site that mimics intact marshes in its physical characteristics, plant and animal life, and ability to perform valued functions such as trapping sediment, cycling nutrients and purifying water. A substantial portion of the world’s tidal marshes has been lost to diking, draining and dredging; in the United States alone, over 800,000 hectares, or one-quarter of the 1932 total acreage, were destroyed between 1932 and 1954 (Queen 1977). Recognition of the value of marshes has led to greater efforts to protect existing sites and to restore degraded tidal marshes (Mitsch and Gosselink 2000). Practitioners have developed a sequence of steps that is standard for most tidal wetland restoration projects: identification of goals, restoration of physical factors, revegetation, recruitment of animal communities, monitoring and continued management. Despite substantial efforts to restore tidal marshes, questions persist about best practices and even if restoration of full functionality is ever possible. What follows is an overview of key terms, theories, and practices in tidal marsh restoration.
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<Figure 1>
DEFINITIONS AND OVERVIEW
Tidal marshes are the intertidal zone characterized by emergent, herbaceous vascular plants and soft sediments. Tidal marshes occur in sheltered regions of bays, estuaries, and the seaward reaches of rivers, where wave action is mild enough that seeds can settle and root before being removed by the water (Chapman 1977). Tidal marshes occur along a gradient of salinity from freshwater, through brackish marshes, to salt marshes (Odum 1988). <Figure 2> Tidal wetlands in the tropics and subtropics are dominated by woody vegetation and are considered a distinct ecosystem from tidal marshes, referred to as mangrove swamps. Because of the differences in vegetation type, mangrove restoration is not covered in this article. Tidal marsh restoration, creation and enhancement Tidal marsh restoration is the alteration of a degraded marsh to resemble an intact marsh in its physical and biological characteristics. It is one type of wetland restoration, which in turn is part of the larger field of environmental restoration. Restoration refers to the re-establishment of an ecosystem type, such as a tidal marsh, where it occurred prior to degradation. Creation is the practice of constructing an ecosystem in a new location, and enhancement is the manipulation of an ecosystem to augment valued services (Spieles et al. 2006). Restoration can be used more broadly to refer to all three of these activities. See also: Restoration Ecology Original tidal marshes and causes of degradation An original tidal marsh (also called a reference or intact marsh) refers to a site that has not been significantly altered by human activity, though it is probable that humans affect all marshes to some degree. Wetlands are often diked or drained to make the land suitable for agriculture or construction, or dredged to widen waterways for boat traffic. Other human impacts on wetlands include water pollution, damming of freshwater sources (changing sedimentation rates and degree of salinity), introduction of invasive species, and waterfowl hunting (Keddy 2000). See also: Land Reclamation, Polder Ecosystem services of tidal marshes Tidal marshes are valued by human society for the ecosystem services they provide, such as: • Biodiversity: Tidal marshes support vegetation and wildlife, including many endangered species and economically valuable species such as waterfowl, fish, shellfish, muskrats and alligators (which are harvested for their pelts) (Mitsch and Gosselink 2000). • Storm abatement: tidal marshes buffer inland areas from ocean storms (Mitsch and Gosselink 2000). • Nutrient cycling: reactive nitrogen, a pollution problem, is converted to benign N2 in wetlands. Sulfur, another pollution problem, is trapped in wetland sediments (Mitsch and Gosselink 2000). • Water quality: apart from nitrogen and sulfur, tidal marshes trap pollutants (Hwang et al. 2006a,b). See also: Wetlands: wetland functions, Nitrogen pollution, Sulfur: environmental impact Wetland protection Since the 1970s there has been a dramatic upsurge of interest in tidal marsh restoration, driven by recognition of the value of such ecosystems and enforced by government regulation. In Europe, wetlands are protected by the European Union’s Habitats Directive (Pethick 2002). In the United States, wetlands are protected under the Clean Water Act, Section 404, which stipulates no net loss of wetlands (Mitsch and Gosselink 2000). This law permits the destruction of wetlands only when the Army Corps of Engineers or State deems an activity unavoidable. Governments in Canada, Australia, New Zealand and China have also acted to reduce wetland loss (Mitsch and Gosselink 2000). Mitigation Mitigation is the practice of allowing wetland degradation on the condition that the responsible party restore or create equivalent wetland functions elsewhere in the watershed. The United States requires compensatory mitigation for permitted activities under section 404 of the Clean Water Act (Mitsch and Gosselink 2000). The practice of mitigation has stimulated great debate in conservation circles. Mature marshes display levels of biodiversity and ecosystem services that are difficult, if not impossible, to restore on a short time scale (Mitsch and Wilson 1996). Given the uncertain outcomes of restoration projects, critics argue that the net result of compensatory mitigation is loss of wetland functions (Zedler 1999). The practice of mitigation banking attempts to eliminate some of the risk in trading existing for restored sites. In mitigation banking, wetlands are restored, created or enhanced in advance of wetland destruction. An organization can then purchase “credits” from the mitigation bank to compensate for its destructive activities elsewhere (EPA 1995). Mitigation banks are intended to create large, high quality marshes that take advantage of economies of scale in their management (EPA 1995). THEORETICAL FRAMEWORK OF TIDAL MARSH RESTORATION Trajectories Implicit in the notion of restoration, defined as “the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed.,” (SER 2004) is the assumption that humans can push ecosystems along a trajectory leading to a predictable endpoint. Practitioners of wetland restoration often assume that the function of restored sites will progress steadily towards that of intact wetlands, as shown in figure 3, depicted in a guide to wetland restoration published by the U.S. Environmental Protection Agency. However, other practitioners debate if tidal wetland restorations will proceed along smooth trajectories of increasing function (Zedler and Callaway 1999). For example, studies have found that soil organic matter did not accumulate with time in restored marshes (Zedler and Callaway 1999, Minello and Webb 1997); another study showed that animal assemblages in restored marshes did not become more similar to reference wetlands over time (Spieles et al. 2006). <Figure 3> PROCESS OF TIDAL MARSH RESTORATION The following presents a synopsis of the typical steps in a tidal marsh restoration project (summarized from Zedler 2001, Broome et al. 1988). Step One: Establish goals for the project The ultimate objective of tidal wetland restoration is to create a site that resembles an original marsh in its physical and biological structure, and in its ability to provide valued ecosystem services. The immediate objective is usually to establish some quantifiable characteristic in the restored site by which the success of the project can be evaluated. Managers can use historical records or local reference wetlands to set benchmarks for a project. Common measurements used to determine the success of a project are: • Hydrology: amount of tidal flow, time of submersion, structure of tidal channel network (Simenstad and Thom 1996); • Sediments: target marsh elevation, sediment quality adequate to support native plants (Simenstad and Thom 1996); • Biota: a given mean percent cover of native vascular vegetation is the most common index of a project’s success (Spieles et al. 2006). The abundance of an endangered organism is another common criterion (Fellows 2001). The hydrology, sediments and vegetation are often referred to as the “structure” of a site and are the most common benchmarks for success because they are important, quantifiable and simple to measure (Zedler and Callaway 1999). Focusing on hydrology, sediments and vegetation or endangered species as the sole measures of restoration success assumes that when certain structural characteristics of a tidal marsh are put in place, the valued functions will also return. Studies which have measured a range of structural characters (such as hydrology, sediments, and vegetation), as well as so-called “functions” (such as animal communities, nutrient cycling, soil organic matter accumulation and water purification) have not found consistent positive correlations between all factors (Zedler and Callaway 1999). For example, Simenstad and Thom (1996) found that, although a restored tidal wetland in the Puget Sound in the northwest USA met structural criteria and supported large populations of birds and fish, the population density of infaunal (soil-dwelling) invertebrates and percent soil organic content remained low over the course of the study, while primary productivity fluctuated unpredictably. Another study of two adjacent estuarine marshes in North Carolina, one original and one restored, found that the original marsh performed denitrification 44 times faster (Thompson 1995). Whether function follows structure, how to restore marsh functions, and if it is even possible to recreate all the processes of an intact wetland, are subjects of continued controversy in the field (Zedler and Lindig-Cisneros 2000). In addition to researching past or reference conditions, current conditions at the degraded site must be taken into account when establishing realistic goals. For example, sites that will be constructed on dredge spoils (sediments that are scooped up to clear waterways, and are often used to recreate marshes) may lack the soil nutrients necessary for marsh plants to grow (Haltiner et al. 1997). Sites that have lost their upland portion to development will not support certain high marsh plant species called peripheral halophytes, nor certain animal species such as beetles and salt marsh mice that require dry refuges during high tides (Goals Project 2000). Step Two: Physical structure Tidal marshes lay between mean sea level and the highest tide line, and are characterized by soft sediments, gradual slopes, tidal flow, and networks of channels. Restoration projects should attempt to recreate these parameters. Hydrology: The first step in any wetland restoration is often to remove a dike so the land will again be alternately covered and exposed by the tides. However, restoring tidal flow is often not as simple as merely removing an embankment. When drained of water, marsh soils are exposed to oxygen, and the large pools of soil organic matter that accumulate in waterlogged, anoxic conditions begin to decompose. As a result, the elevation of the soil subsides. When a dike is removed, the soil may now lie below mean sea level, and thus will be inhospitable to marsh plants (Adam 1990). Dredge spoils may be used to raise the surface of sediments to an appropriate height, but care must be taken to select soils that are hospitable to native plants (Callaway 2001). In addition, project managers should grade the marsh substrate to have a gradual slope, ideally 1-3%, to allow wave energy to dissipate over a large area (Broome et al. 1988). Microtopography has a surprisingly large effect on plant communities. Small elevation differences of a few centimeters create microhabitats that maintain much of the plant diversity in tidal marshes, preventing just a few plants from dominating the system (Larkin et al. 2006). Another key feature of tidal marshes is tidal creeks, networks of channels that crisscross the marsh plain. Some restoration projects have constructed only a few, straight channels restricted to subtidal elevations (Callaway 2001). Natural channel systems are typically quite dense, with numerous branches, and traverse a range of elevations from the subtidal to the high intertidal. Studies have found that fish favor the smaller branches off the main channels (Visintainer 2006), that channels are vital for maintaining plant species diversity (Sanderson 2000), and that the shallow intertidal reaches of channels are vital foraging areas for birds (Josselyn 1983). Sediments Proper soil is essential to plant growth, and yet many restoration and creation projects have employed sandy and silty dredge spoils, to the detriment of marsh function (Callaway 2001). At Sweetwater Marsh in San Diego, California, a wetland was constructed using silt that was unable to retain nitrogen, a vital plant nutrient; as a result, a healthy plant community never established (Boyer et al. 2000). If soil is needed, the ideal source is a local, dredged wetland. In general, soil should be evaluated for appropriate texture, compaction, salinity, organic content, nutrient concentrations, and pH (Callaway 2001). Step Three: Introduction of biota Vegetation There are two schools of thought on how to plant in a newly restored or created marsh: the “designer” and “self-design” approaches (Mitsch and Wilson 1996). The designer approach distributes the species of seedlings and cuttings in the approximate zones where they occur in reference wetlands. By contrast, the self-design approach plants the regional assembly of species throughout the marsh, or allows plants to naturally colonize the site, or allows for a combination of planting and recruitment. The concept behind self-design is to allow natural selection to winnow out the survivors, determining the ultimate species composition and zonation of the site (Mitsch and Wilson 1996). However, given that marsh species are often restricted to a 10-20 cm vertical range, it can be a waste of resources to plant individuals outside their preferred habitats (Sullivan 2001). In general, seedlings are more sensitive to desiccation and salinity than adult plants, so the best time to plant is usually during the rainy season (Broome 1988). A factor which is gaining more attention recently is the importance of different genotypes of species in ecosystem functioning. For example, Spartina alterniflora, a dominant plant in salt marshes on the east coast of the USA, shows local adaptation between and within marshes (Proffitt 2002). As a result, it is best to take seeds or cuttings from many plants to endow a new marsh with vital genetic diversity, but at the same time, source plants should be from nearby populations that are adapted to local conditions (Sullivan 2001). Fauna It is relatively rare to actively introduce animal species to tidal marshes (Williams and Desmond 2001). It is often assumed that animals are mobile and will colonize an area of suitable habitat once the plants are established. However, those benthic invertebrates that are sessile and lack a planktonic larval stage have extremely limited dispersal. In such cases active transplants may help develop the faunal community (Levin et al. 1996). Even relatively mobile species can be prevented from dispersing to a site if it is not connected to source populations; restoring marshes close to existing marshes should promote the recruitment of animals to the site (Sacco et al. 1994). Step Four: Monitoring and management Monitoring programs aim to assess if a restored site has attained the concrete goals laid out at the beginning of a project. In the case of compensatory mitigation in the United States, monitoring is generally stipulated for 5-10 years (Zedler and Callaway 1999). Adaptive management is regarded by many restoration practitioners as the best approach for coping with the uncertainties of project outcomes (Zedler 2001). In adaptive management, the restoration project is initially set up with experiments to test for the best practices at a site, and the practices at a site can be modified to pursue the best strategy. EXTERNAL LINKS EPA wetlands site http://www.epa.gov/owow/wetlands/ EPA’s mitigation banking factsheet http://www.epa.gov/owow/wetlands/facts/fact16.html EPA’s National Estuary Project http://www.epa.gov/owow/estuaries/habitat/index.html Map and listings of National Estuary Projects, including tidal wetland restoration activities http://www.epa.gov/owow/estuaries/pivot/habitat/hab_fr.htm Society for Environmental Restoration http://www.ser.org/ Society of Wetland Scientists http://www.sws.org/ Coastal America http://www.coastalamerica.gov/
Map of European wetlands from the European Environment Agency http://dataservice.eea.europa.eu/atlas/viewdata/viewpub.asp?id=2219