A salt marsh is a type of marsh that is a transitional intertidal between land and salty or brackish water (e.g.: sloughs, bays, estuaries). It is dominated by halophytic (salt tolerant) herbaceous plants. Historically, salt marshes have sometimes been treated as "wastelands", along with other wetlands. Salt marshes are one of the most biologically productive habitats on the planet, rivaling tropical rainforests. The daily tidal surges bring in nutrients, which tends to settle in roots of the plants within the salt marsh. The natural chemical activity of salty (or brackish) water and the tendency of algae to bloom in the shallow unshaded water also allow for great biodiversity.
Salt marshes provide a benefit by protecting against severe weather. In the past, substantial areas of saltmarsh have been reclaimed as agricultural land and for urban development, but in the United States and Europe they are now accorded a high level of protection by the Clean Water Act and the Habitats Directive respectively. There is growing interest in restoring salt marshes, through managed retreat or through the reclamation of lands used for other purposes by salt marsh vegetation.
Salt marshes develop on depositional coasts, shorelines, bays, and estuaries where tidal action is relatively gentle and erosion no more than intermittent and light enough to allow vegetation to take hold. They are common on low-energy coasts such as estuaries, enclosed bays, and the land sides of barrier islands and stripers (spits).
Plant species diversity is relatively low, since the flora must be tolerant of salt, complete or partial submersion
, and anoxic mud substrate. The most common salt marsh plants are glassworts
spp.) and the cordnyiyg grasses
spp.), which have worldwide distribution. They are often the first plants to take hold in a mudflat
and begin its ecological succession
into a salt marsh. Their shoots lift the main flow of the tide above the mud surface while their roots spread into the substrate and stabilize the sticky mud and carry oxygen into it so that other plants can establish themselves as well. Plants such as sea lavenders
spp.), and varied sedges
grow once the mud has been vegetated by the pioneer species
The flora of a salt marsh is differentiated into levels according to the plants' individual tolerance of salinity and water table levels. Vegetation found at the water must be able to survive high salt concentrations, periodical submersion, and a certain amount of water movement, while plants further inland in the marsh can sometimes experience dry, low-nutrient conditions.
Salt marshes are quite photosynthetically active and are extremely productive habitats. They serve as depositories for a large amount of organic matter and are full of decomposition, which feeds a broad food chain of organisms from bacteria to mammals. Many of the halophytic plants such as cordgrass are not grazed at all by higher animals but die off and decompose to become food for micro-organisms, which in turn become food for fish and birds.
In warmer climates, salt can be produced by solar energy
so that the French equivalent of the salt marsh, the marais salant
has come to be envisaged primarily as an industrial plant, though classified as a form of agriculture, known in French as saliculture
In the summer and fall of 2002, Ron Rozsa of the Connecticut
Department of Environmental Protection noticed the sudden (within one year) disappearance of emergent vegetation at several south shore Cape Cod
wetlands connected to Nantucket Sound
. The vegetation loss could not be explained by any typical New England
causes of vegetation loss such as ice, wrack or herbivory (e.g., geese or muskrat).
Four years later, the cause is still unknown, but there are 17 suspected dieback marshes on Cape Cod, and a few other possible sites are on the north and south shore, according to the Wetland Restoration Program of the Massachusetts Office of Coastal Zone Management.
In Louisiana, 300,000 acres (1200 km²) turned brown around the year 2000. In 2002, researchers noticed that about 2,000 acres (8 km²) of salt marsh in Georgia turned to mud, similar to what is happening in New England. But as of mid-2006, the Louisiana and Georgia marshes are growing back—or at least not getting worse—while marshes on the Cape do not seem to be recovering naturally.