move slowly up the basin (Chabreck 1970). As the 

 basin's hydrologic regime changes, all of the habitats 

 are affected. In the Calcasieu Basin dredging of a deep 

 ship channel has accelerated saltwater intrusion (part 

 3.6. ."i). In the Mermentau Basin, on the other hand, 

 control structures have prevented saltwater intrusion 

 and the basin is becoming progressively fresher (part 

 3.6.3). 



Brackish 

 n\ai"sh 



Fresh marsh 

 and 



Figure 4-4. Water sahnities (mean ± standard devia- 

 tion) in five natural habitats in the Louisi- 

 ana portion of the Chenier Plain (Cha- 

 breck 1972). 



4.2.1 A FUNCTIONAL OVERVIEW OF WET- 

 LANDS 



The structural and functional similarities of the 

 natural wetland habitats far outweigh their dif- 

 ferences. The trophic interactions in each habitat have 

 much in common, although the animal species in- 

 volved at each trophic level change with habitat. The 

 flow diagram in figure 4-5 summarizes the inter- 

 acting parameters characteristic of emergent wetlands. 

 The many arrows crossing the system boundaries indi- 

 cate the high degree of interaction between adjacent 

 habitats. The whole system is shown as a net pro- 

 ducer, since organic export to adjacent estuarine 

 waters is characteristic of wetlands. The sun's energy 

 is the ultimate source of all plant production, but the 

 hydrologic regime (shown as H in the diagram), the 

 available nutrients, and salts are the primary regulators 

 of this production. 



Three groups ofplants are identified in the model. 

 Most of the energy trapped is by the emergent 

 vascular plants which dominate the marsh. The 

 periodically inundated stems of these plants and the 

 marsh fioor support a vigorous growth of diatoms and 

 other algae which contribute up to 10% of the net 

 primary production (Gosselink et al. 1977). Sub- 

 merged grasses (such as widgeongrass) and phyto- 

 plankton produce additional food. These three pro- 

 ducer groups are distinguished by different rates of 

 production, quality of production, and the relation- 

 ship of production to biomass (turnover rate). In 

 general, die algae, because of their high protein con- 

 tent, are a more nutritious food source to grazers than 

 emergent grasses are, and have a more rapid turnover 



rate. This makes their contribution to the food web 

 more important than their biomass would seem to 

 indicate. 



The food web sliown in the model (fig. 4-5) in- 

 cludes a detritus compartment and three consumer 

 groups. Because emergent grasses consist mostly of 

 ceDulose and other compounds that are not digestible 

 by most consumers, they are eaten by a diverse group 

 of scavenging animals only after the dead tissue is 

 enriched by bacteria and other microbes. This is the 

 most important food pathway in marsh habitats. 

 However, grass seeds and tubers, submerged grasses, 

 and many algae are consumed directly by insects, 

 crustaceans, birds, and even mammals such as the 

 muskrat and nutria. These scavengers and grazers, 

 in turn, are prey for such carnivorous animals as 

 hawks, some fish, and predaceaous insects. 



While the simple food web in the model is con- 

 ceptually useful, in reality trophic relationships are 

 not nearly so clear. For instance, puddle ducks 

 apparently eat vegetation to store carbohydrates for 

 migration, but switch to high protein animal diets at 

 nesting time. Other trophic relationships are dis- 

 cussed in sections dealing with individual populations. 

 For this overview it is sufficient to understand that 

 several trophic levels exist, but that most food energy 

 is processed through the detritus-scavenger pathway. 



Because primary production is high, wetlands 

 support large numbers of migratory andnonmigratory 

 animals. These include commercially important shell- 

 fishes (shrimp, oyster, blue crab), finfishes (Gulf 

 menhaden) and mammals (muskrat, nutria), as well as 

 species prized by sportfishennen (spotted sea trout, 

 redfish, fiounder) and hunters (ducks, geese). 



Other equally important marsh processes are not 

 as closely related to the food web. These include 

 (from Gosselink et al. 1974): 



1 . The value of the marsh as a storm buffer and 

 flood water reservoir; 



2. The sediment filtering and trapping action of 

 emergent grasses and filamentous algae; 



3. The ability of marshes to purify flood waters 

 by removal of wastes, nutrients, and toxins. 



4.2.2 ROLE OF HYDROLOGY IN WETLAND 

 HABITATS 



Wetland Habitats. The properties, distribution, 

 and circulation of water are considered by many to 

 be tlie most important controlling features of wet- 

 land habitats. Water is the vehicle for movement of 

 biota into and out of wetlands. Water also controls 

 marsh productivity and species richness, peat for- 

 mation, organic export, and the inorganic nutrient 

 flux into the marsh. Water level determines the 

 accessibihty of small marsh ponds to aquatic con- 

 sumers and the usefulness of marshes to waterfowl. 

 This section will document the importance of the 

 fluctuating hydrologic regime that sustains wetland 

 habitats. 



157 



