Distribution and Composition of Coastal Marshes

Salt and brackish marshes are found in all southeastern states with marine coasts.  The species composition and biology of Southeastern coastal marshes, especially in regards to plants, may vary geographically, however the basic physical conditions that define the ecosystems share similar characteristics across the region.  In general, they are defined by periodic tidal inundation, dominance of salt-tolerant herbaceous plants and a rate of net primary production that is among the highest of all the world’s ecosystems.  Recorded salinity varies widely, from 1 to over 50 ppt in Louisiana salt marshes.  Net primary production (above-ground plant material) may also be highly variable.  Although production has been estimated in only a few Florida salt marshes, the range spans from 130 to 2500 grams dry mass per square meter per year (g m ^–2 yr ^-1).

At some sites, Southeastern coastal marshes are dependent on burns to reduce woody vegetation such as saltbush, (Baccharis halimifolia) and wax myrtle (Myrica cerifera) (Wade 1991), however at other sites high salinity may be sufficient to preclude most woody plants.  On the Atlantic coast, where tides and wave action are often strong, marshes occur primarily in protected areas associated with barrier islands.  Along the Gulf of Mexico, where wave energy is generally low, marshes often occur fronting the ocean.  In Louisiana, the Mississippi Delta adds complexity to coastal marsh classification.  In that state, large areas of marsh may span gradients from salt water to brackish and intermediate marshes before finally ending in freshwater marshes.  A gradation from high salinity (>30 ppt), to brackish to freshwater is common in many areas (Figure 1 in Odum et al. 1984).  This gradation is correlated with many differences in species composition and ecological processes (Odum 1988). 

Vegetation

Depending on site location and physical characteristics (especially salinity), the dominant plant species may include cordgrass (Spartina spp.), rush (Scirpus spp.), needlerush (Juncus) and/or saltgrass (Distichlis spicta) (Gosselink 1984, Odum et al. 1984, Stout 1984, Wiegert and Freeman 1990).  Species composition is determined, in part, by geography and local elevation that, in turn, influences length of time of inundation and salinity.  Across the region, sub-dominant plant species often include saltgrass and saltwort (Batis maritime).  Coastal marshes east of Lousiana are almost exclusively dominated by black needlerush (J. roemerianus).  Along the Atlantic coast, as far north as North Carolina, there is a range of salt marsh vegetation that may be dominated by smooth cordgrass (Spartina alterniflora) or black needlerush.  In the delta coastal marshes of Louisiana, smooth cordgrass and saltmarsh cordgrass (S. patens) may dominate a saline marsh, with black needlerush and salt grass as sub-dominants.  In brackish delta marshes, saltmarsh cordgrass is dominant and the saltmarsh dominant plants listed above often represent sub-dominants.  The intermediate delta marshes are also dominated by saltmarsh cordgrass but the sub-dominants may vary. (Gosselink 1984).

Although, by definition, salt marshes are largely devoid of woody vegetation, brackish sites often include shrubs such as saltbush (Baccharis halmifolia).  Along the upper (high elevation) edges of marsh areas there may be scattered individuals of other woody species, including marsh elder (Iva frutescens).  In general, salt marsh algae are an order of magnitude less productive than salt marsh vascular plants and microalgae may be important nutritionally for many resident invertebrate (Montague and Wiegert 1990). 

Although, the vegetation structure of salt marshes is very similar, there can be distinct differences in vegetation composition.  Correct identification of dominant plants may be important for effective evaluation of fire effects.  

Mangrove swamps, the other common coastal ecosystem in the southeast, is generally limited to southern Florida.  The transition between salt marshes and mangrove swamps is at approximately 30⁰ N.  North of this point, winters are usually too cold for mangroves. 

Animals

There are numerous animal species (both aquatic and terrestrial) associated with salt and brackish marshes (Gosselink 1984, Stout 1984, Wiegert and Freeman 1990).  Invertebrates include various species of crabs and mollusks and many species of terrestrial arthropods; grasshoppers and plant hoppers are common herbivores.  Although no species is usually locally abundant, invertebrates may be a species- rich group in salt marshes.  In North Carolina, a smooth cordgrass marsh supports more than 109 species of herbivorous insects (Wiegert and Freeman 1990).

Native species of amphibians are rarely found in association with salt or brackish marshes, however salt-tolerant leopard frogs have been collected in Florida (references in Montague and Wiegert 1990).  In addition, the introduced marine toad (Bufo marinus) is sometimes found in association with salt marshes of South Florida.

Several reptile species, including the American alligator (Alligator mississippiensis), use salt and brackish marshes but are not restricted to them.  In addition there are a few coastal marsh reptile species that are of conservation concern.  Subspecies of the secretive salt marsh snake (Nerodia fasciata) are endangered, as is the diamondback terrapin (Malaclemys terrapin). 

Salt marshes also provide habitat for fur-bearing species (raccoon, mink, muskrat and, the introduced nutria) and other small mammals such as rice rats (Gosselink 1984, Stout 1984, Wiegert and Freeman 1990).  However, in many states, the economic value of furbearers is now of less significance than in the past, when small mammal trapping was more common.  In Atlantic coastal marshes dominated by smooth cordgrass, the rice rat is the only year round resident mammal species.  

Coastal marshes provide important habitat for many types of waterfowl and wading birds, especially species of herons and egrets (Gosselink 1984, Stout 1984, Wiegert and Freeman 1990).  In addition, subspecies of seaside sparrows are of concern over much of the coastal southeast.  Habitat loss, due in part to permanent impoundment, could help to explain declining populations but marsh burning has also been a focus of concern. Unfortunately, there are scant data on seaside sparrows prior to their decline.  Other secretive marsh birds, especially species of rails and wrens, are also in need of study.

There are many sources of information for general accounts of these ecosystems, as well as species lists and summaries of many aspects of natural history.  The delta marshes of Louisiana are reviewed by Gosselink (1984). Stout (1984) profiled related ecosystems adjoining the northern Gulf of Mexico, from the Pearl River in Mississippi to Cedar Key in Florida.  Wiegert and Freeman (1990) did the same for Atlantic coastal marshes from North Carolina to Florida.  Additional information on Florida sites is contained in Montague and Wiegert (1990).  Chabreck (1972) provided a detailed description of broad marsh patterns in Louisiana and Wharton (1977) is often cited for succinct descriptions of the natural ecosystems of Georgia, including coastal marshes.