CHAPTER 5 

 NICHE DIVERSITY, TROPHIC INTERACTIONS, AND COMMUNITY STRUCTURE 



5.1. HABITAT-SPECIFIC ASSOCIATIONS 



The Apalachicola estuary, as an 

 ecosystem, can be defined as a series of 

 habitats with associated assemblages of 

 organisms. Such assemblages (or communi- 

 ties) live in the same general habitat, 

 compete for space and food, and are part 

 of the highly complex trophic structure of 

 the river-bay system. The dimensions of a 

 given community are difficult to define 

 precisely because the component 

 populations vary considerably in their 

 distribution and community function in 

 space and time. However, selected factors 

 can be used to characterize the various 

 estuarine assemblages. Sources of primary 

 productivity, habitat features, the 

 physical and chemical environment 

 (including pollutants), modes of 

 reproduction and recruitment, feeding 

 interactions, predator-prey relations, and 

 competition are some of the features that 

 shape the estuarine communities. 



The distribution of most of the 

 estuarine assemblages may be partitioned 

 into the following habitats: marshes, 

 seagrass beds, litter associations, oyster 

 bars, and subtidal unvegetated (soft- 

 sediment) areas. Many of the long-term 

 biological studies in the Apalachicola 

 estuary have concentrated on the macro- 

 invertebrates (benthic, epibenthic) and 

 fishes that are found in these areas. 



S.1.1. Marshes 



The marshes, which include complex 

 patterns of tidal channels and small 

 creeks, provide food and habitat for a 

 number of organisms in the Apalachicola 

 estuary (Table 18). Marsh complexes 

 include insects, mollusks, crustaceans, 

 fishes, birds, and mammals. Topminnows of 



various species are dominant in such 

 areas. Many species that are important to 

 the sports and commercial fisheries of the 

 region spend at least part of their life 

 histories in the estuarine marshes. Such 

 species include blue crabs, penaeid 

 shrimp, large-mouth bass, lepomids, 

 striped mullet, spotted and sand seatrout, 

 and anchovies. Few species spend their 

 entire lives within the marshes, however, 

 and the marsh habitat is best 

 characterized as a nursery for migratory 

 species during summer and fall months. 



5.1.2. Seagrass Beds 



The distribution of grassbeds in the 

 Apalachicola estuary (Figure 19) is the 

 result of a number of environmental 

 controlling factors. Even though it is 

 limited to only about 10% of the aquatic 

 area by the high turbidity and 

 sedimentation associated with the river, 

 this habitat's productivity is high. 

 Grassbed productivity is also limited by 

 water temperature, salinity, and the 

 activity of certain invertebrates. 

 However, grassbeds also have an effect on 

 certain water quality indices. Various 

 studies in East Bay (Livingston 1978; 

 Purcell 1977) indicate that water quality 

 factors such as dissolved oxygen and pH 

 are higher in the grassbeds than in 

 associated mudflats. 



The oligohaline grassbeds of East Bay 

 are dominated by tapeweed ( Val isneria 

 americana ), a freshwater species. Other 

 species found in conjunction with tapeweed 

 are Potamogeton pusil lus , Ruppia maritima 

 (locally dominant in western bayous of 

 East Bay), Cladophora sp., and Halophila 

 engelmanni . In recent years, some parts 

 of East Bay are being taken over by the 

 Eurasian watermilfoil ( Myriophyllum 



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