MDPR: 182,000 ha (449,540 acres), or 

 more than 5% of the total area in 1978 

 (Table 5). MDPR salt marshes represent 

 one of the largest contiguous salt marsh 

 zones in the world (Figure 35) . 



There have been a number of studies 

 of plant production in salt marshes in 

 the MDPR (e.g., de la Cruz 1974; Kirby 

 and Gosselink 1976; Hopkinson et al. 

 1978a; White et al. 1978). Studies of 

 animal ecology and soil and nutrient 

 dynamics in the salt marsh have also 

 been carried out, but most of this 

 research has examined single aspects of 

 salt marsh function. The salt marsh 

 ecosystem in Georgia was described in 

 terms of an energy budget by Teal 

 (1962), and a tentative carbon budget of 

 the salt marsh system in the MDPR was 

 constructed by Day et al. (1973). These 

 studies were attempts to synthesize 

 existing information on salt marsh 

 function, but the absence of quantita- 

 tive data on many parameters necessi- 

 tated the use of many simplifying 

 assumptions . 



Much additional information on salt 

 marshes has been developed since 1973, 

 especially with respect to microbio- 

 logical processes, but many uncertain- 

 ties still remain in our understanding 

 of the MDPR salt marsh habitat. The 

 companion technical report (Costanza et 

 al. 1983) contains the detailed salt 

 marsh matter and energy flow diagram, 

 input-output table, and accompanying 

 documentation. 



The most conspicuous feature of the 

 salt marsh is the broad expanse of salt 

 marsh cordgrass ( Spartina alternif lora ) 

 that comprises 61% of the emergent 

 vegetation in terms of percent cover. 

 Other important salt marsh macrophytes 

 are black rush ( Juncus roemerianus ) , 

 saltgrass ( Distichlis spicata ) , and 

 saltmeadow cordgrass ( Spartina patens ) . 



Various authors have studied the 

 primary production of marsh macrophytes. 

 A value of 2,050 g dry wt/m 2 /yr was 

 estimated for the aboveground net pro- 

 duction of an average MDPR salt marsh. 

 Studies have pointed out the possible 

 significance of belowground production, 



however, (Valiela et al . 1976; Stout 

 1978; and Gallagher and Plumley 1979). 

 It was estimated for the present study 

 that belowground net production by marsh 

 macrophytes contributed 5,970 g dry 

 wt/m 2 /yr, a value almost three times 

 greater than aboveground production 

 (Costanza et al. 1983). This estimate 

 is not as good as aboveground production 

 figures, however, and our understanding 

 of salt marshes in the MDPR will be 

 markedly improved when belowground pro- 

 duction rates (and the subsequent fate 

 of the carbon produced) is carefully 

 measured. Another relatively unknown 

 parameter in terms of marsh production 

 is the amount of low molecular weight 

 organic carbon that leaches out of 

 leaves during the growing season. Cur- 

 rent aboveground production estimates do 

 not take this leachate into account and 

 thus may be conservative. 



Marsh microflora, particularly 

 benthic diatoms on the mud surface, and 

 epiphytic algae on marsh grass leaves 

 and stems contribute significantly to 

 the productivity of the salt marsh. It 

 is estimated that all of these plants 

 combined produce 750 g dry wt/m 2 /yr. 



The food web of the salt marsh is 

 based primarily on detritus. Although 

 some salt marsh grass is grazed while 

 living by invertebrates such as insects, 

 and mammals, most of the carbon produced 

 by the grass is eaten after it partially 

 decomposes in the soil and water column. 

 For example, consumption of living pro- 

 ducers by the periwinkle ( Littorina 

 irrorata ) accounts for only 4% of the 

 total dietary intake of this snail 

 (Alexander 1976). Grazing of live salt 

 marsh cordgrass is low because the grass 

 is fibrous, high in cellulose and lig- 

 nins, and low in nitrogen. The major 

 macroconsumers in the salt marsh are 

 crabs, mussels, snails, insects, birds, 

 and muskrats, nutria, and raccoon. 

 Nektonic organisms are omitted here 

 because tidal streams are not included 

 as marsh, and nekton are considered 

 residents of the estuarine open water 

 habitat. 



Microbial forms are pivotal in the 

 marsh habitat, in that they regulate in 



74 



