Salt Marshes

Coggeshall Marsh, Prudence Island Courtesy: NOAA

Rhode Island salt marshes are found along the shores of salt ponds, the Narragansett Bay estuary, small embayments (such as Allin's Cove in Barrington), and estuarine rivers (such as the Narrow River estuary). Our salt marshes provide nursery grounds and foraging habitat for hundreds of species of fish, shellfish, birds, and mammals. Fish of all sizes, from mummichogs to striped bass, hunt in creeks and ponds. Quahogs and oysters live beneath the surface, while mussels, fiddler crabs, and snails occupy intertidal areas. Many kinds of birds visit the marsh to feed on the fish and invertebrates: osprey and herons, ducks of all sorts, and mosquito-eating sparrows that nest in the marsh. In addition to their habitat value, salt marshes serve as natural pollution treatment systems by filtering out pollutants before they reach our coastal waters. The location of salt marshes between our developed coastal communities and the waters of the state also provides a buffer during storms and flooding.

Seventy-five percent of commercial fish species depend on estuaries for their primary habitat, spawning grounds, and nursery areas. In Rhode Island, the role that salt marshes play in our economy is evidenced by our 75 million dollar commercial fishery and a recreational fishery valued at 150 million dollars. The sweeping vistas afforded by the low lying salt marsh landscape contribute immeasurably to the beauty and serenity of Rhode Island's coastline, as well as our tourism and outdoor recreation industry, which is valued at 2 billion dollars on Narragansett Bay alone.

History and Impacts

It is estimated that 60% of Rhode Island's salt marshes have been filled with mud and sand dredged during navigation projects or waste material derived from upland sources (Save The Bay 2002). Downtown Providence was once known as the Great Salt Cove, prior to filling and conversion to uplands. Marshes can be completely filled or they can be partially filled, altering the tidal exchange of water, and impacting vegetation communities that rely on twice-daily flooding. Often the result of such changes in elevation and flooding is the invasion by undesirable species such as Phragmites australis (common reed). Phragmites is very tolerant of disturbed sites, and can rapidly overtake such areas.

Phragmites rapidly invades salt marshes that have been disturbed or filled. Courtesy: D. Yozzo, Barry Vittor & Associates

Construction of dikes, roads and rail crossings has resulted in the degradation of many marshes in Rhode Island. Restriction of tidal flow by installation of small culverts or drainage pipes under roads and rail beds leads to changes in salinity and alteration of the natural vegetation community due to a reduction in duration and frequency of tidal flooding. Phragmites, which is tolerant of these altered conditions, especially reduced salinity, often invades rapidly in areas that have been culverted or diked. Phragmites out-competes native salt marsh vegetation, and reduces local biodiversity. Some 1200 of the existing 3700 acres of salt marsh in Narragansett Bay are impacted by Phragmites and other invasive plant species (Save The Bay 2002)

Fish communities of salt marshes also suffer from road/rail infrastructure, as they rely on the natural tidal cycle to maintain populations in salt marshes. Marsh resident fish species, such as killifish (Fundulus spp.) spawn in concert with the tidal cycle, timing their spawning activity to coincide with the highest Spring tides, due ensure deposition of eggs in the highest portion of the marsh (Taylor et. al. 1979). When natural tidal cycles are interrupted, or reduced, killifish spawning success is impaired. Tidal restrictions can reduce the amount of habitat available for estuarine-dependent fish that travel up into tidal creeks in search of food.

Mosquito ditching has impacted many marshes in Rhode Island. Mosquito ditches are very straight, narrow channels that were dug to drain the upper reaches of salt marshes. Historically, it was believed that ditching marshes would control populations of mosquitoes that breed there. It is now known that ditching, in fact, drains standing water which support populations of mosquito-eating fish (e.g., killifish), leading to increases in mosquitoes. These fish are an important prey item for wading birds (herons and egrets), as well as larger, predatory fish species. Mosquito ditching alters natural patterns of groundwater drainage, which alters plant community composition, and nutrient cycling.

Polluted runoff from adjacent uplands can degrade salt marshes. Runoff from roads and other paved surfaces, and nutrient-rich runoff from fertilized lawns, agricultural areas, and septic systems can degrade marshes by encouraging growth of Phragmites and other invasive species. Forested buffer zones between populated areas and salt marshes have diminished as population growth in coastal areas increases. Approximately 58% of Narragansett Bay's marshes are impacted by polluted runoff. Some 30% of the Bay's marshes have inadequate or non-existent buffer zones (Save The Bay, 2002).

Healthy vs. Degraded Salt Marshes

An example healthy marsh with a vast meadow of Spartina, upland buffer, and no road crossings Courtesy: Coastal Resources Mangement Council

Healthy Salt Marsh
The typical vegetation of a salt marsh (smooth cordgrass and salt hay grasses) are a beautiful green in the summer and interspersed with purple sea lavender. Fiddler crabs can be seen scurrying along the intertidal mudflats at lower tides and fish dart through the tide pools and ditches. Sharp tailed sparrows and red winged blackbirds nest in tidal marshes. A great egret or a glossy ibis can be seen circling the marsh, while black ducks slowly wade through the marsh. The healthy salt marsh offers more than just a picture of beauty and wildlife; it is actively enhancing water quality by filtering sediments, nutrients, heavy metals, and other toxins from upland runoff. This benefit helps keep our state's waters clean and available for swimming and other recreational activities. In addition to the filtering processes, salt marshes are also intercepting waters during storms that could cause expensive, damaging flooding of residential and commercial properties.

An example degraded marsh that has a large stand of Phragmites and hydrologic restriction Courtesy: Save The Bay

Degraded Salt Marsh
Today many of us would be surprised to learn how extensively salt marshes and other wetland types used to occur in Rhode Island. For instance, Providence was one area where salt marshes were prevalent and have since been filled in to make way for population centers. Obviously, areas such as these cannot be reclaimed as salt marsh habitats. But there are other sites in our watersheds where tidal flows have been restricted, invasive plants are taking over, and sedimentation is changing the hydrology of the marsh. These types of impacts to a salt marsh result in lower biodiversity, a decrease in flood abatement and erosion benefits, and provide potential mosquito breeding habitats.

Salt Marsh Restoration

As early as the 1950s, studies had shown that changes to the tidal flushing, or hydrology, of a salt marsh would degrade its quality as habitat, and that the restoration of historic hydrology could restore the habitat value of the marsh. In New England, where many salt marshes have been hydrologically altered by road and dike construction, dredged material disposal, and ditching, salt marshes have been a major focus of coastal habitat restoration efforts. Looking back on these projects after a decade or more, many of them have been very successful. The science and technology of salt marsh restoration are becoming fairly well established.

Salt marshes are complexes of specialized plants and animals living in a low-energy, intertidal environment. Hydrology, the precise way in which the marsh is flooded by the tide, dictates which species can survive in any particular part of the marsh, and therefore defines the entire marsh ecosystem.

Low marsh is flooded by nearly every tide. The dominant vegetation in a New England low marsh is usually smooth cordgrass. Ribbed mussels and fiddler crabs are some of the more common animals. High marsh is flooded only by the highest high tides. In New England, the dominant high marsh vegetation is generally salt hay grass, black grass, or spike grass. Common animals include deer and a wide variety of birds, including great blue herons.

The simplest form of salt marsh restoration involves re-establishing historic hydrology where it has been lost (see Restoration Methods for more details). In New England, roads and railways have often been built through marshes. Many times, culverts were placed in the tide creeks at the road crossings. The culverts may have been too small to begin with, or may have collapsed over time. The resulting tidal restrictions prevent most tides from reaching the marsh inland of the road. The marsh becomes a brackish, rather than tidal system; Phragmites, replaces the salt marsh vegetation; and the marsh habitat becomes severely degraded. In these instances, the installation of a culvert or culverts large enough to allow the full tidal range to return may be all that is needed to restore the marsh. The salinity of the tides will kill the Phragmites, and salt marsh vegetation and fauna will return over a number of years. Two completed projects in Rhode Island that primarily involved hydrologic restoration are highlighted on this Web site: the Little Mussachuck Creek Restoration and the Sachuest Salt Marsh Restoration.

In other cases, the marsh itself may have been filled, often as a disposal site for marine sediments dredged from navigational channels. In these instances, it is not enough merely to restore hydrology because the surface of the marsh is too high to allow the tide to flood it. In these instances, marsh elevations must be re-established in order to restore a salt marsh ecosystem. These types of restoration projects tend to be more difficult and expensive as more construction is required, and the logistics of earth-moving in a marsh can be challenging. Nevertheless, some very successful projects of this type have been accomplished, such as the Galilee Salt Marsh Restoration in Narragansett, and the Allen Harbor Restoration in North Kingstown, Rhode Island.

Salt Marsh Restoration Case Study: Galilee Bird Sanctuary

Galilee Bird Sanctuary Courtesy: NOAA

The Galilee Bird Sanctuary is a 130-acre salt marsh owned by the state of Rhode Island. The sanctuary is located east of the port of Galilee and is bounded by the Galilee Escape Road to the north and Sand Hill Cove Road to the south. During the l950s, dredged material from the Port of Galilee was deposited in parts of the marsh, filling in a tidal channel and significantly altering the natural tidal action.

During Hurricane Carol in 1954, coastal flooding in the area was so severe that local residents were trapped, unable to evacuate their homes. As a result, in 1956 the state built the Galilee Escape Road, which greatly improved public safety in Galilee but caused additional harm to the salt marsh by further restricting tidal flushing. These changes transformed the marsh into a dense thicket of Phragmites with very little habitat value for birds, fish, or shellfish.

In 1991, the R.I. Department of Environmental Management (DEM), which manages the marsh as a wildlife refuge, began a project to restore the ecological productivity of the marsh by reestablishing tidal flushing. DEM's Division of Fish and Wildlife organized a partnership of federal, state, and non-governmental agencies; developed a restoration plan; oversaw construction; and monitored the recovery of the salt marsh. The team regraded the marsh, cut new tidal channels, and installed large culverts under the Galilee Escape Road to allow the tide to return to the marsh. The culverts are outfitted with self-regulating tide gates that allow normal tides in and out of the marsh, but prevent extreme tides from flooding coastal homes.

Tide gates at Galilee Courtesy: NOAA

Since the completion of construction in 1997, the ecology of the marsh has begun to return. Fish and wildlife have responded to the restoration in dramatic fashion, as finfish recolonized the tidal creeks within days following opening of the tide gates. Waterfowl, including the American black duck, use the restored marsh extensively for nesting, feeding, and resting during migration. In total, approximately 84 acres of salt marsh habitats and 14 acres of tidal creeks and ponds were restored. Project partners included the U.S. Army Corps of Engineers, DEM Mosquito Abatement, the University of Rhode Island, R.I. Coastal Resources Management Council, R.I. Department of Transportation, Ducks Unlimited, and U.S. Fish and Wildlife Service.

Related Links

Save The Bay's Web page on Salt Marsh Restoration

Coastlines June 1997 - Setting the Stage for Salt Marsh Restoration in Narragansett Bay

Galilee Salt Marsh virtual tour - Discovery of Rhode Island Coastal Environments

References

Save The Bay. 2002. Restoration projects throughout the Narragansett Bay watershed. (http://www.savebay.org/bayissues/restoreprojects.htm).

Save The Bay, Inc., People for Narragansett Bay. 2001. Save The Bay Web site: What is Habitat Restoration? (http://www.savebay.org/).

Taylor M.H., G.J. Leach, L. DiMichele, W.M. Levitan, and W.F. Jacob. 1979. Lunar spawning cycle in the mummichog, Fundulus heteroclitus (Pisces: Cyprinodontidae). Copeia 1979:291-297.

This site was created through a partnership of the: Coastal Resources Management Council Narragansett Bay Estuary Program Save The Bay®