Scarborough Marsh is a prime example of a New England back-barrier salt marsh (Figure 1). Maine’s largest tidal marsh, Scarborough Marsh is approximately 3,000 acres in size and accounts for 15% of the state’s total salt marsh area. Virtually the entire marsh is owned by the Maine Department of Inland Fisheries and Wildlife, while almost all of the surrounding uplands are in private ownership.

Salt marshes are common features of sheltered estuaries. Estuaries are aquatic ecosystems that are partially enclosed by land where saltwater from the ocean mixes with freshwater from upland rivers and surface runoff. Salt marshes and mudflats are common estuarine wetlands. In the Gulf of Maine, salt marshes develop in a range of salinity from that of seawater, about 34 parts per thousand (ppt) of salt, to approximately 18 ppt. Brackish marshes are characterized by salinity ranging from 0.5 - 18 ppt. Freshwater tidal marshes are located where the salinity averages less than 0.5 ppt yet tides still affect the movement of water.

Tidal marshes form in low-lying coastal areas that are protected from excessive winds, waves, and currents. These "low energy" environments allow for the deposition of sediments suspended in tidal waters. Subsequently, marsh plant communities develop on this sediment base. Each of these two processes, the deposition of sediment material and the colonization by tidal marsh plants, reinforce one another. Once plants are established they trap additional sediments, and the increased deposition of sediments raises the marsh elevation, allowing expansion of the tidal marsh. Along the Maine coast, sea level has risen slowly over the past 4,000 to 5,000 years. Throughout this period salt marshes have maintained themselves at the tidal elevations necessary for plant growth through the accretion of sediments filtered from tidal waters and the formation of peat from plant fragments and sediments. Two common species are Smooth Cordgrass (Spartina alterniflora), which grows in areas flooded by daily tides, and Salt Meadow Grass (Spartina patens), which grows at a higher elevation in irregularly flooded areas.

Scarborough Marsh contains a variety of habitats that support different plants and animals. Some of the more important habitat types are described below.

Low marsh is flooded twice daily by tidal action. Smooth Cordgrass is the dominant plant in salt and brackish low marshes. Low marsh typically occurs as a sloping fringe between the high marsh and a tidal creek or mudflat.

High marsh is flooded irregularly by above-average tides. Salt Meadow Grass and Black Grass (Juncus gerardii) are the dominant plants in most high marshes. In brackish marshes with a strong freshwater influence, plants such as Narrow-leaf Cattail (Typha angustifolia) or bulrushes (Scirpus sp.) may dominate. High marsh is usually level and occurs between the low marsh and uplands. Healthy high marsh can be seen on the seaward of the Eastern Road, along the Nonesuch and Libby Rivers seaward of Black Point Road, and on the seaward side of Pine Point Road at Jones Creek.

Pannes are shallow "ponds" that form within high marsh. Flooded periodically by spring tides (the monthly above-average tides associated with full moon and new moon), pannes provide an abundance of food for waterfowl and migrating shorebirds. Good examples of pannes can be seen on the seaward side of Route 1 and along the Scarborough Marsh Nature Center trail on the west side of Pine Point Road.

Tidal creeks, open water, and tidal flats are also important components of the marsh ecosystem. Open water can be defined as a permanently flooded (i.e., below mean low water) water body greater than 100 meters (330 feet) wide. Tidal creeks are narrower, and may or may not have water at low tide. Tidal flats are nearly level to gently sloping unvegetated areas located within the intertidal zone.

Variation in topography, geology, tides, sediment supply, wave exposure, and rate of sea-level rise along the Maine coast lead to the development of different marsh types. The three basic types are coastal/back barrier marshes, finger marshes, and fringe marshes. The three marsh types can be determined visually from maps (see Figure 2) and are described below.

Scarborough Marsh is a back-barrier salt marsh, formed behind the protective sand beach at Pine Point and the more stable shoreline at Prouts Neck. Back barrier marshes are most common southwest of the Sheepscot River where most of Maine’s sand beaches are found. Finger marshes are often found at the head of bays, while fringe marshes are found along protected shorelines in estuarine reaches of large rivers (coves, indentations, small tributaries, meanders) or at the toe of an eroding bluff. These types are more common in midcoast and Down East Maine.

                   A                                               B                                              C

Figure 2. (A) Back-barrier Marsh; (B) Finger Marsh; and (C) Fringe Marsh

Tidal marshes can be described in terms of the ecological functions they perform and the values they hold for society. Many ecological functions and societal values are closely related (e.g. wildlife habitat and recreation). Tidal marsh ecosystems are formed and persist through a combination of geological, hydrological and biological processes or functions. Several of these functions have been described in light of the tangible benefits they provide to the human community, directly or indirectly. Scientists recognize several important ecological functions of tidal marshes, including:

• Shoreline anchoring - The accretion of peat and sediment maintains marsh elevation as sea level rises and buffers the upland shoreline against the erosive action of the open water.
• Storm Surge Protection - The resistance to water flow presented by marsh vegetation slows the movement of water over the marsh, thereby helping to protect low-lying uplands and erodable shorelines during storms.
• Water Quality Maintenance - Pollutants often enter aquatic systems attached to sediment particles. Many of these particles are deposited on the marsh, limiting their transport to other ecosystems. Some pollutants may then bind with soil particles and become unavailable for uptake by plants or animals.
• Wildlife, Finfish, and Shellfish Habitat - The rapid growth rates of salt marsh grasses form the base of a highly productive food web. Salt marshes are used for food and shelter by a diverse animal community, including many species of birds, finfish, shellfish and other invertebrates. The linkage between salt marsh productivity and the health of the Gulf of Maine remains to be studied, but it is likely that coastal marshes are important contributors to the productivity of the greater Gulf of Maine ecosystem.

These ecological functions have a tremendous financial value. Two-thirds of commercial shellfish and finfish landed in the U.S. depend on coastal wetlands for nursery and breeding habitat or on forage fish that breed in our coastal wetlands (Gosselink et al. 1974). The estimated total income for the harvest and processing of finfish and shellfish in Maine in 1993 was $462 million, resulting in twenty-two thousand jobs (Wilson 1993). Recreational fishing, hunting, wildlife watching, and boating in coastal wetlands also contribute significant economic value.

Humans have depended on tidal marshes in Maine for millennia. Native Americans harvested birds, fish and shellfish from tidal marshes for thousands of years. In an area dominated by extensive forests, uplands adjacent to salt marshes became preferred sites for European settlement because the marsh grasses provided abundant fodder for their livestock.

Today, tidal marshes are valued by society for a variety of reasons. Scarborough Marsh offers some of Maine’s best opportunities for bird watching, coastal canoeing and kayaking, sport fishing, and waterfowl hunting. Salt marshes are the major native grasslands of coastal New England, and their open, grassy expanses and coastal vistas are aesthetically attractive to many. As ecosystems that have maintained themselves for thousands of years, they provide excellent outdoor classrooms for the teaching of basic ecological concepts.

Human activities have significantly altered Scarborough Marsh and other tidal marshes. The most significant threat continues to be roads and railroads that form barriers to the free flow of tidal water. These barriers limit the amount of water reaching the inner marsh, and hence the salinity and amount of sediment available for marsh accretion. The worst type of tidal restriction is the tide gate, which is designed to prevent salt water from reaching sections of the marsh. The marsh will then decompose as it becomes less saline, resulting in a lower marsh surface elevation on the inland side of the restriction. If the marsh surface subsides more than a few inches it becomes impossible to fully restore the native plant community that existed prior to the restriction. The spread of invasive plants such as Common Reed (Phragmites australis), often abetted by tidal restrictions, also threatens the ecological integrity of the marsh. Past ditching for agriculture or mosquito control and direct fill of the marsh also add to the cumulative impacts to the marsh, while ongoing development in the surrounding uplands affects water quality and wildlife habitat. All these factors have shaped the character of the Scarborough Marsh as we see it today.

¹ Most of the material in this section was adapted from Bryan et. al. 1997.