different times of the year. Estabrook 

 (1973) noted that grazing zoopiankton also 

 may control phytoplankton productivity 

 since experiments removing zooplankton and 

 net plankton enhanced nannoplankton 

 productivity greatly. The possibility 

 exists that competition for nutrients 

 among various species also is an important 

 determinant of relative phytoplankton 

 dominance. 



Among the zooplankton, copepods are 

 dominant. The copepod Acartia tonsa 

 constitutes 95.5% of total zooplankton in 

 East Bay, 68.2% in Apalachicola Bay and 

 19.8% in coastal waters (Edmisten 1979). 

 Salinity and temperature control the 

 composition of zooplankton communities in 

 the estuary. Populations of Acartia vary 

 inversely with distance from the mouth of 

 the Apalachicola River and are 

 concentrated in Apalachicola Bay. 

 Temperature is associated with significant 

 (p < 0.01) differences in Acartia numbers. 

 Salinity significantly (p < 0.01) affects 

 the overall relative abundance of the 

 dominant populations. Edmisten (1979) 

 showed that temperature, salinity, station 

 and month had a multiple r value of 0.775. 

 In East Bay, Acartia numbers (as well as 

 zooplankton numbers and biomass) peak 

 during periods of high salinity. Thus, 

 temperature usually determines overall 

 numbers in the bay system, while salinity 

 determines their spatial distribution at 

 any given time. The response to midrange 

 salinities explains the nonlinear 

 (parabolic) relationship of Acartia with 

 salinity,. It appears that other 

 organisms can successfully complete with 

 Acartia at higher and lower salinities. 



Life history strategies of various 

 nektonic estuarine species depend to some 

 degree on spatial/temporal gradients of 

 substrate type, salinity, food 

 availability, and energy flow. The 

 spatial distribution and abundance of 

 brief sguid ( Lolliguncula brevis ) is 

 determined to a considerable degree by 

 salinity and temperature (Laughlin and 

 Livingston 1982). Optimal salinities 

 range between 25 and 30 ppt. Squid tend 

 to congregate near the passes during 

 summer and fall periods of high salinity. 

 Distribution within the estuary is 

 associated with the distribution of 

 zooplankton in the bay. Population trends 



of squid followed long-term (9-year) 

 salinity trends that, in turn, were 

 associated with climatic features. There 

 were sharp decines in squid abundance 

 during periods of low salinity. 



Overall, attempts to correlate 

 patterns of species abundance with 

 individual physical, chemical, and 

 productivity variables have not been 

 entirely successful. A multiple 

 regression analysis of individual 

 population densities with combinations of 

 independent variables indicates that such 

 components accounted for less than 50% of 

 the population variability (Table 23). No 

 single set of physical conditions 

 explained population variation through 

 time. While factors such as temperature, 

 salinity, productivity, and water quality 

 characteristics are important determinants 

 of general habitat availability, it is 

 clear that other factors, presumably 

 biological in nature, may be important to 

 our understanding of the processes that 

 determine the community structure of the 

 Apalachicola Bay system. 



5.3. TROPHIC RELATIONSHIPS AND FOOD-WEB 

 STRUCTURE 



Community structure is determined in 

 part by predator-prey interactions, 

 especially among dominant estuarine 

 populations. Comprehensive studies of the 

 feeding habits of dominant fishes 

 (Sheridan 1978; Sheridan and Livingston 

 1979) and invertebrates (Laughlin 1979) 

 have been carried out (Figure 37). 

 Pelagic anchovies feed preimarily on 

 calanoid copepods throughout their lives. 

 Seventy percent of the diet of young 

 anchovies (standard length (SL), 10-39 mm) 

 is composed of these copeoods. Larger 

 fish (SL 40-69 mm) eat mysids, insect 

 larvae and juvenile fishes. A seasonal 

 progression of food item consumption 

 follows trends of available prey species. 

 The Atlantic croakers progress through a 

 series of distinct ontogenetic trophic 

 stages. Young fish (SL 10-30 mm) eat 

 insect larvae, calanoid copepods, and 

 harpacticoid copepods. "Midrange fish (SL 

 40-99 mm) consume detritus, mysids, and 

 isopods; larger fish (SL 100-159 mm) eat a 

 high proportion of juvenile fishes, crabs, 

 and infaunal shrimp. Croaker at all 

 stages eat polychaete worms. Spot, which 



83 



