Tidal Mesohaline and Polyhaline Marshes
Communities in this ecological group are salt marshes characterized by very low species diversity and low plant stature. Mesohaline conditions comprise salt concentrations between 5 and 18 ppt, whereas polyhaline conditions vary from 18 to 30 ppt. Salt marshes range along the Atlantic and Gulf coasts from Maine to Texas. In Virginia, they occur both on lower stretches of tidal rivers and creeks in the inner Coastal Plain and both shores of the Chesapeake Bay (where generally only mesohaline conditions attain) and on extensive non-riverine flats, where salinity may range from mesohaline to polyhaline. These latter communities are best developed on the Eastern Shore (Accomack and Northampton Counties), where they occupy several thousand hectares of essentially flat plains, especially on the Atlantic side. Some of the higher and more interior salt marshes are only irregularly tidal, but are compositionally indistinguishable from diurnally tidal communities.
Riverine marshes are strongly dominated by saltmarsh cordgrass (Spartina alterniflora), often in association with big cordgrass (Spartina cynosuroides) or saltmarsh bulrush (Bolboschoenus robustus, = Scirpus robustus). Non-riverine salt marshes are characterized by extremely low diversity and are dominated by saltmarsh cordgrass, saltmeadow cordgrass (Spartina patens), saltgrass (Distichils spicata), or some combination thereof. Vegetation composition and stature generally reflect elevation of substrate, which influences salinity and frequency and duration of inundation. Low salt marsh, dominated by the "short form" of saltmarsh cordgrass, occupies lower surfaces and forms extensive mosaics on the seaside of the Eastern Shore. Saltgrass and saltmeadow cordgrass are the characteristic species of high salt marsh, which typically occurs on slightly elevated surfaces where tides may be less regular and where soils may concentrate salts.
Although tidal mesohaline and polyhaline marshes are typically characterized by extremely low diversity, some high salt marshes, or salt meadows, occasionally have a moderately diverse assemblage of halophytes, including woody glasswort (Sarcocornia pacifica), seaside oxeye (Borrichia frutescens), sea lavender (Limonium carolinianum), salt marsh fleabane (Pluchea odorata var. odorata), annual glassworts (Salicornia virginica and Salicornia bigelovii), salt-marsh pink (Sabatia stellaris), salt marsh false foxglove (Agalinis maritima), black grass rush (Juncus gerardii), and narrow-leaved loosestrife (Lythrum lineare). Another common community type features strong dominance by black needlerush (Juncus roemerianus), which often forms extensive stands. The abundance of this species, which also occurs in oligohaline marshes, may have increased as a result of contemporary reductions in fire frequency in salt marshes.
References: Clovis (1968), Coulling (2002), Harvill (1967), Levy (1983), Perry and Atkinson (1997), The Nature Conservancy (1997).
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A large Eastern Shore salt marsh complex on Custis Creek in Accomack County. Photo: Kevin E. Heffernan / © DCR Natural Heritage.
Saltmarsh cordgrass (Spartina alterniflora) stand on College Creek in James City County (Colonial National Historical Park). Photo: Irvine Wilson / © DCR Natural Heritage.
Low salt marsh, with near-monospecific dominance by the "short form" of saltmarsh cordgrass (Spartina alterniflora), on Gargathy Bay, Accomack County. Photo: Irvine Wilson / © DCR Natural Heritage.
A patch of black needlerush (Juncus roemerianus , with dark stems) in a salt marsh fringe along King Creek near its confluence with the York River in York County. Photo: Gary P. Fleming / © DCR Natural Heritage.