Tidal Oligohaline Marshes
This group contains primarily graminoid-dominated wetlands of slightly brackish zones along tidal rivers and streams of the Coastal Plain. Oligohaline conditions are defined as salt concentrations between 0.5 and 5 ppt, although pulses of higher halinity may occasionally occur. Vegetation in this group occurs from Maine to Georgia. In Virginia, big cordgrass (Spartina cynosuroides) is the most characteristic and abundant species and often forms extensive, tall stands, particularly along edges of the main tidal channels. Associates include a mix of species characteristic of freshwater marshes, such as dotted smartweed (Persicaria punctata) and arrow-arum (Peltandra virginica), and species more tolerant of higher salinities, such as swamp rose-mallow (Hibiscus moscheutos) and seashore mallow (Kosteletzkya pentacarpos). Other species indicative of oligohaline conditions (when occurring in marshes) include halberd-leaved tearthumb (Persicaria arifolia), swamp barnyard grass (Echinochloa walteri), and swamp dock (Rumex verticillatus). Some oligohaline marshes contain dense colonies of shoreline sedge (Carex hyalinolepis) or, more commonly, narrow-leaf cattail (Typha angustifolia), the latter of which may be increasing in extent as the result of fire exclusion and eutrophication. Diversity generally decreases as halinity increases, but some communities of mixed composition, particularly those of low stature, may support more species than many tidal freshwater marshes.
Dredge spoils and other disturbed areas often support dense, nearly monospecific colonies of common reed (Phragmites australis ssp. australis ); this highly aggressive, invasive subspecies constitutes a serious threat to tidal marshes throughout the Coastal Plain. The diurnally tidal communities in this group are compositionally distinct from the group of more diverse oligohaline marshes in extreme southeastern Virginia that are subject to irregular wind tides.References: Coulling (2002), Megonigal and Darke (2001), Perry and Atkinson (1997), The Nature Conservancy (1997).
Click on the images below to open a larger image in a separate window.A mixed tidal oligohaline marsh with big cordgrass (Spartina cynosuroides), swamp rose-mallow (Hibiscus moscheutos, with white flowers), and seashore mallow (Kosteletzkya pentacarpos, with pink flowers). Sweet Hall Marsh on the Pamunkey River, King William County.Photo: Phil Coulling / © DCR Natural Heritage.
Tidal Oligohaline Marsh dominated by saltmeadow Cordgrass (Spartina patens) and Olney threesquare (Schoenoplectus americanus) at the mouth of Chotank Creek, King George County (Chotank Creek Natural Area Preserve).Photo: Gary P. Fleming / © DCR Natural Heritage.
In Virginia, big cordgrass (Spartina cynosuroides) is the most characteristic and abundant species of tidal oligohaline marshes. Jamestown Island, James City County (Colonial National Historical Park).Photo: Gary P. Fleming / © DCR Natural Heritage.
Bull-tongue arrowhead (Sagittaria lancifolia var. media) blooming among big cordgrass (Spartina cynosuroides) in a tidal oligohaline marsh on Jamestown Island, James City County (Colonial National Historical Park). Photo: Gary P. Fleming / © DCR Natural Heritage.
Olney threesquare (Schoenoplectus americanus , with brown fruits) and dotted smartweed (Persicaria punctata) in a tidal oligohaline marsh at the mouth of College Creek, James City County (Colonial National Historical Park). Photo: Gary P. Fleming / © DCR Natural Heritage.