Salt Marsh 



An ecosystems diagram for the salt marsh is presented in Figure 3-5. A 

 coastal salt marsh is often considered to be a pioneer stage of successional 

 development (Allan, 1950; Penfound, 1952), and it is therefore expected to be 

 characterized by high primary productivity and relatively low species diversity 

 (Odum, 1969). Smooth cordgrass ( Spartina alterniflora ) is typically dominant 

 and may produce 1,500-3,000 g dry wt/m^ per year. This large biomass is the 

 major source of detritus which accumulates annually and supports the fauna of 

 the marsh community; the excess production is exported to the adjacent water 

 system via tidal flushing (Day et al., 1973). 



This highly productive ecosystem is dependent upon tidal forces for import 

 and export of materials and energy. Vegetative zonation and succession are 

 governed by elevation, salinity, drainage/aeration, and edaphic conditions - 

 all of which are directed by tidal fluxes (Daiber, 1974). Freshwater inputs 

 may be significant also; their magnitude varies geographically and seasonally. 



The primary components of the salt m.arsh ecosystem include stored sediment, 

 nutrients, salt, heat, and detritus; standing crops of flora (grasses, epiphytes, 

 and phytoplankton) ; and fauna (primary, secondary, and tertiary consumers). 



Nutrients, sediment, and heat are moved to and from adajcent systems by 

 tidal flushing forces. In contrast, freshwater runoff is responsible only for 

 an inflow of materials and energy. 



Tides . Periodic components of tides are due to gravitational forces. 

 Along the Gulf of Mexico the annual tidal ranges resulting from these forces 

 are small, usually ranging between 0.3 and 0.6 m (1 and 2 ft) (Marmer, 1954). 

 The effects of winds can be significant in altering the areas of inundation in 

 a marsh. Prevailing winds from the southeast are common in summer, and strong 

 north winds occur intermittently in fall and winter months. Periodic hurricanes 

 represent another seasonal phenomenon which may strongly influence this eco- 

 system. Thus, gravitation, wind, and elevation act to determine the frequency 

 of inundation in the salt marsh, which, in turn, controls the general hydro- 

 period of any given area. 



The frequency of inundation is an important regulator of gaseous exchange 

 between plants and their environment and the chemical state of nutrients in 

 the soil. Although smooth cordgrass cannot withstand complete submergence for 

 extended periods (Redfield, 1972), a well developed vascular system allows this 

 species a significant competitive advantage in the regularly flooded salt marsh 

 (Teal and Kanwisher, 1966). Permanently saturated or flooded soils are character- 

 ized by a lack of oxygen. The resulting low redox potentials favor the reduced 

 (and more available) form of nutrients. Periodic exposure to air increases 

 rates of oxidation; this phenomenon provides an adequate supply of plant- 

 available nutrients to support the high rates of growth in smooth cordgrass 

 (Redman and Patrick, 1965; Brupbacher et al., 1973; De Laune et al., 1976). 



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