Coastal salt marsh, also termed tidal salt marsh, is the dominant, mostly herbaceous vegetation of seacoast mudflats, especially outside the tropics and subtropics, including around estuaries of all sizes, coastal plain, and deltas, and characterizing vegetation along huge bays, inlets, and fjords in subpolar and polar zones. For just the United States, salt marsh extends, interrupted only by beaches, from New Jersey to northern Florida along the Atlantic Ocean, rimming the Gulf of Mexico, and at scattered, nonrocky stretches from Washington to San Diego. These are incredibly productive biological systems, and are major feeding and nesting grounds of birds. In tropical and subtropical latitudes, coastal salt marsh is instead mangal, i.e., mangrove vegetation.
Like mangal, salt marsh is populated by halophytes, plants that can live under saline conditions. In the lowest portion of a typical marsh (lower marsh), especially one that is open to the sea, plants generally are flooded twice each day at high tide. These areas require plants to be able to withstand regular inundation by saltwater. Higher there may be an upper marsh zone, where inundation may not be as often, for example, less than one time per day. If inundation is uncommon and evaporation is intense, of if a marsh is closed from direct contact with seawater, a different type of halophytic vegetation may occur, such as salt flat succulent. High evaporation in dry coastal locations may produce muddy salt flats so toxic that no plants will grow there, at soil salt concentrations exceeding one hundred parts per thousand, i.e., three times that of ocean water. At the other end of the scale, river discharge of large volumes of freshwater into the upper salt marsh may have make some zones weakly brackish, supporting yet a different set of wetland plants.
The following illustrate coastal salt marshes of California:
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