Regularly flooded marsh in the southeast United States is dominated by stands of smooth 

 cordgrass (Spartina alterniflora), which may at first appear to lack zonation. However, work by 

 Teal (1958), Valiela, Teal, and Deuser (1978), and others indicates total biomass varies 

 considerably within the low marsh, ranging from zones of tall 5. alterniflora along active creek 

 banks to stunted or short 5. alterniflora stands away from creeks and drainage channels. The tall 

 S. alterniflora may be caused by a combination of factors, including more nutrients, a higher 

 tolerance for the reductions in oxygen that result from subtle increases in elevation along levees 

 (DeLaune, Smith, and Patrick 1983), and differences in drainage created by variations in the 

 porosity of sediment. The zone where S. alterniflora grows is thought by many to be limited in 

 elevation to mean high water. This is probably too broad a simplification according to Redfield 

 (1972), who emphasized that the upper boundary of the low marsh is, at best, indistinct. 



High marsh, in contrast, consists of a variety of species. These include Salicornia spp. 

 (glassworts), Distichlis spicata (spikegrass), Juncus spp. (black needlerush), Spartina patens (salt- 

 marsh hay), and Borrichia frutescens (sea ox-eye). Teal (1958) reports that Juncus marsh tends to 

 be found at a slightly higher elevation than the Salicomia/Distichlis marsh. 



The high marsh can also be distinguished from low marsh on the basis of sediment type, 

 compaction, and water content. High-marsh substrate tends to be firmer and dryer and to have a 

 higher sand content. Low-marsh substrate seldom has more than 10 percent sand (except where 

 barrier-island washover deposits introduce an "artificial" supply) and is often composed of very 

 soft mud. Infrequent flooding, prolonged drying conditions, and irregular rainfall within the high 

 marsh also produce wide variations in salinity. In some cases, salt pannes form, creating barren 

 zones. But at the other extreme, frequent freshwater runoff may allow less salt-tolerant species, 

 such as cattails, to flourish close to the salt-tolerant vegetation. These factors contribute to 

 species diversity in the transition zone that lies between S. alterniflora and terrestrial vegetation. 



By most reports, low marsh dominates the intertidal areas along the southeast (Turner 

 1976), but the exact breakdown can vary considerably from place to place. Wilson (1962) 

 reported 5. alterniflora composes up to 28 percent of the wetlands in North Carolina, whereas 

 Gallagher, Reimold, and Thompson (1972) report for one estuary in Georgia that the same 

 species covers 94 percent of the "marsh" area. Low marsh is thought by many to have a 

 substantially higher rate of primary productivity than high marsh (Turner 1976). Data presented 

 in Odum and Fanning (1973) for Georgia marshes support this notion. However, Nixon (1982) 

 presents data for New England marshes that indicate above-ground biomass production in high 

 marshes comparable to that of low marshes. Some data from Gulf Coast marshes also support 

 this view (Pendleton 1984). 



Potential Transformation of Wetlands 



The late Holocene Oast several thousand years) has been a time of gradual infilling and loss 

 of water areas (Schubel 1972). During the past century, however, sedimentation and peat 

 formation have kept pace with rising sea level over much of the East Coast (e.g., Ward and 

 Domeracki 1978; Due 1981; Boesch et al. 1983). Thus, apart from the filling necessary to build 

 the city of Charleston, the zonation of wetland habitats has remained fairly constant there. 

 Changes in the rate of sea level rise or sedimentation, however, would alter the present ecological 

 balance. 



If sediment is deposited more rapidly than sea level rises, low marsh will flood less frequently 

 and become high marsh or upper transition wetiands, which seems to be occurring at the mouths 

 of some estuaries where sediment is plentiful. The subtropical climate of the southeastern United 

 States produces high weathering rates, which provide a lot of sediment to the coastal area. 

 Excess supplies of sediment trapped in estuaries have virtually buried wetlands around portions 

 of the Chesapeake, such as the Gunpowder River, where a colonial port is now landlocked. 



If sea level rises more rapidly in the future, increased flooding may cause marginal zones 

 close to present low tide to be under water too long each day to allow marshes to flourish. Unless 



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