Apalachicola estuary reflects the response 

 of hundreds of species to a complex 

 combination of physical, chemical, and 

 biological factors. Physical control, 

 together with productivity features, 

 recruitment patterns, predator-prey 

 interactions, and competition for various 

 resources determine to a considerable 

 degree the form and functions of the soft- 

 sediment communities in the Apalachicola 

 Bay system. Because the majority of the 

 research in the Apalachicola Bay system 

 has been carried out with the fishes and 

 macroinvertebrates of the soft-sediment 

 estuarine habitat, the interrelationships 

 of the dominant features of these 

 biological systems will be treated in a 

 more detailed fashion below. 



5.2. PHYSICAL CONTROL OF BIOLOGICAL 

 PROCESSES 



For some time, ecologists have argued 

 about the relative importance of physical 

 and biological control of aquatic 

 populations and communities. Clearly, the 

 problem is extremely complex, based on the 

 fact that each species is a product of a 

 given habitat while also having an input, 

 through predation and competition, to the 

 community. It is generally agreed that 

 temperate estuaries such as the 

 Apalachicola system are highly productive 

 and physically unstable in space and time. 

 Temperature and salinity have a major 

 influence on the form and processes of the 

 estuarine biota in such a system. At the 

 same time, various populations interact 

 with each other and their environment with 

 almost continuous feed-back to the system 

 as a whole. 



evaluation of hypotheses derived from 

 observational data can then be used to 

 determine the processes that define and 

 ultimately control the observed structural 

 components of the system. 



Various attempts have been made to 

 delineate the relationships of physical 

 and biological variables in the 

 Apalachicola estuary (Livingston 1975, 

 1976b, 1979, 1982b; Livingston and Loucks 

 1978; Livingston et al. 1974, 1976b, c, 

 1978; Mahoney and Livingston 1982; Meeter 

 and Livingston 1978; Meeter et al. 1979). 

 Most analyses indicate that Apalachicola 

 River flow has a major influence on the 

 physical and biological relationships in 

 the estuary. For example, statistical 

 analysis of the principal physico-chemical 

 variables (Table 21) indicates that the 

 main factor or component could be called 

 "river flow." This river flow is 

 associated with low salinity, increased 

 color and turbidity (and reduced Secchi 

 readings), and reduced chlorophyll _a. 

 River flow alone explained 32% of the 

 total variance and about half of the 

 variance explained by the four factors. 

 Average bay values of major nutrients vary 

 seasonally; high nutrient concentrations 

 are found during high (winter) river 

 discharge and low salinity conditions 

 (Table 22). The Apalachicola River 

 controls to a considerable degree various 

 factors such as nutrient and detritus 

 concentrations, salinity, color and 

 turbidity, and other water quality 

 factors. Tn turn, these conditions 

 control the level and pattern of 

 productivity fluctuations in the bay 

 system. 



The timed interactions of multiple 

 physical and biological components of an 

 estuarine system are difficult to 

 differentiate for a variety of reasons. 

 Individual physical events follow 

 different temporal patterns. Often such 

 phenomena are essentially cyclic although 

 "cycle" does not necessarily imply that 

 there is a complete return to a previous 

 condition. Biological responses are not 

 that simple and often follow nonlinear or 

 curvilinear patterns of response to 

 varying controlling factors. Analysis of 

 biological responses requires the initial 

 delineation of key dependent and 

 independent variables. Experimental 



Studies of temperate estuaries 

 indicate that the combination of high 

 primary productivity and extremely 

 variable environmental conditions is often 

 associated with relatively low species 

 richness and diversity and high secondary 

 productivity of a few dominant species. 

 No matter which group of organisms is 

 considered, from phytoplankton to fishes, 

 salinity appears to be the primary 

 regulator of species numbers at a given 

 location in the estuary. Dominants are 

 able to adapt to low or highly variable 

 salinity conditions. Salinity is a major 

 determinant of species richness (S) of 



80 



