Found all along the eastern coast of North America from Newfoundland to Florida, salt marshes occupy the constantly fluctuating border between land and sea, being exposed and flooded twice daily by the tide. They are usually found on the less exposed side of barrier beaches and harbor shorelines, where the wave energy is not as great as on the ocean beach. Salt marshes are not as common on Nantucket as they are elsewhere because most of the island’s shoreline is exposed to the Atlantic Ocean and Nantucket Sound. Well-developed salt marshes can be found in the sheltered waters of Nantucket Harbor at First Point, Second Point, Third Point, Five-fingered Point, Coskata, The Haulover, Quaise, Pimney’s Point, the Creeks, and within Polpis Harbor. Additionally, the west end of the island supports salt marshes at Eel Point, Jackson’s Point, and Hither Creek.
Most of the salt marshes in New England were formed after the retreat of the last glacier approximately 12,000 years ago. As this ice sheet melted, large amounts of fine silt, clay, sand, and other organic sediments were picked up, carried, and deposited in sheltered locations. The seeds of marsh grasses that are adapted to being periodically inundated by the tide germinated and colonized the surface of this rich, organic soil.
The roots of salt marsh plants bind the sediments together to form a firm, peaty substrate that traps organic debris brought in by each tide. As the marsh develops, dead blades of grass that are replaced each year by new growth accumulate on the peat surface and decompose, slowly increasing the elevation of the surface. This substrate forms the foundation for the plants and animals that inhabit the salt marsh. For thousands of years, this process of peat formation has kept pace with the slow rise in sea level that has been occurring since the retreat of the glacier. One of the impacts of climate change is that this process is now unable to keep up with escalating sea level rise.
There are two distinct salt marsh zones that can be readily distinguished by the plants that grow there: the low marsh and the high marsh. The low marsh is the area closest to the water and along tidal creeks that is regularly flooded at high tide. The dominant plant in this zone is smooth cordgrass (Spartina alterniflora). This species, which can grow to four feet in height, is adapted to being submerged in salt water about half the time, based on the tide cycle. Despite its long length, only the tips of the blades are usually visible during the peak of high tide.
As the name implies, the high marsh is further back from the shoreline and is at a higher elevation than the low marsh. This zone is only flooded during the extremely high tides associated with the full and new moons. Here, salt marsh cordgrass is replaced by several other plant species. The most common is salt marsh hay, or saltmeadow cordgrass (Spartina patens), which grows to a maximum of 15 inches in height. This species can be recognized at a distance by its tousled appearance, due to new growth arising from the flat, matted stems of the previous year’s growth. It grows in pure, continuous stands that give the surface of the high marsh a textured appearance. Also found in this zone are saltgrass (Distichlis spicata) and saltmarsh rush (Juncus gerardii). The former species is light green and prefers the wettest portions of the high marsh. The latter species, which occurs in drier areas, is dark green and derives its name from its blackish fruits, which ripen in June and persist into the autumn.
There are many other species of plants that grow on the marsh. Sea lavender, or marsh rosemary, grows in the high marsh near the upland border. Its leaves form a small rosette that grows close to the ground, sending up a flower stalk with many small, lavender flowers in late summer. This species has become rare in recent years because it has been over-collected for use in dried flower arrangements. Salt marsh aster, seaside goldenrod, and seabeach knotweed are examples of other flowering plants that are associated with salt marshes.
Another species that can be found along the upper edge of the marsh is Phragmites, or common reed. This perennial grass can reach 15 feet in height and forms purplish, plume-like seed heads in late summer. Phragmites spreads rapidly by underground rhizomes and out-competes cattails and other marsh plants. The resulting thick, impenetrable stands of reeds provide minimal value for wildlife. Phragmites is especially prolific in salt marshes and other wetland areas disturbed by construction that has altered the daily ebb and flow of the tide. Ditching and filling, two common management practices undertaken to control mosquito populations (with limited success) also disturb marsh soil and make these sites more prone to colonization by this species.
Although Phragmites is native to North America, it has become very invasive across its range within the last fifty years. Research has demonstrated that the native and invasive populations are genetically different and that the invasive strain is responsible for degrading wetlands. Once established, Phragmites is extremely difficult to eradicate without the use of mechanical alterations and herbicides.
The high marsh also provides habitat for several species of shrubs adapted to being occasionally flooded by salt water during storms and the most extreme high tides. Groundsel tree is a sizable shrub with thick, coarsely toothed leaves. It produces whitish bristly flowers in late summer. Marsh elder often grows in thickets along the marsh edge. It has dark green, sharply toothed leaves and produces plain, green flowers in late summer. Both of these species are also called high tide bush because they grow along the border between the upland and where the highest tides reach their peak.
Peat sometimes accumulates faster in some portions of the marsh than in others, creating slight differences in topography. As the high tide recedes, water will often become trapped where the elevation is slightly lower than the surrounding peat. Marsh grasses cannot survive with their roots continuously soaked in salt water, so the vegetation at such sites gradually dies off and decomposes, leaving small salt ponds on the marsh surface. These locations often contain pools of standing, concentrated salt water even when the tide is low and contain a variety of interesting microscopic organisms that have adapted to living under these extremely saline conditions.
Click here to learn more about the Foundation's successful salt marsh restoration project and associated research at Medouie Creek.