biological organization of the estuary. 

 Infaunal macroinvertebrates reach maximum 

 abundance from November through March, 

 although species richness is highest in 

 May. As indicated previously, 

 phytoplankton and zooplankton are abundant 

 during spring months and summer periods. 

 Fish abundance peaks during winter and 

 early spring although fish and 

 invertebrate species richness indices 

 reach their highest level in October. 

 Epibenthic invertebrate abundance, on the 

 other hand, is high during August when 

 penaeid shrimp and blue crabs are 

 prevalent. In general, the dominant fish 

 species, while overlapping in abundance to 

 some degree, tend to predominate during 

 different times of the year; high croaker 

 and spot abundance occurs in winter and 

 early spring, sand seatrout in summer, and 

 anchovies in the fall and early winter. 

 Water column feeders such as anchovies are 

 linked to plankton outbursts and predation 

 pressure from species such as sand 

 seatrout. Benthic feeders occur primarily 

 during periods of detritus/ 

 macroinvertebrate abundance. Croakers and 

 spot feed largely on polychaetes, while 

 blue crabs concentrate on bivalves. 

 Directly or indirectly, most such species 

 take advantage of the detritus that is 

 brought into the estuary by the river. 

 The combination of low salinity, high POM, 

 and low predation pressure contributes to 

 the observed high relative abundance of 

 these species. 



5.4. PREDATOR-PREY INTERACTIONS AND 

 COMMUNITY RESPONSE 



Although productivity trends and 

 habitat characteristics are important 

 factors in the development and control of 

 food web and community structure, 

 biological features such as predator-prey 

 relationships and competition for 

 resources can be extremely important in 

 affecting the biological organization of 

 the estuary. Predation within aguatic 

 associations can lead to changes in 

 relative abundance, species diversity, and 

 other important community indices. 

 Peterson (1979) reviewed factors that 

 relate the impact of predation and 

 competitive exclusion to the response of 

 benthic macroinvertebrates in unvegetated, 

 soft-sediment estuarine habitats. 

 Previous work with various marine assem- 



blages (largely rocky intertidal 

 communities) has indicated that isolation 

 from predation (through manipulative 

 processes such as caging) should lead to 

 increased total density, increased species 

 richness, and restriction of competitive 

 exclusion by particular dominant species 

 (Peterson 1979). According to this model, 

 manipulative predator exclusion should 

 cause simplification of the prey community 

 as a result of enhanced competition due to 

 increased population densities. Various 

 authors have found that soft-bottom 

 associations of benthic macroinvertebrates 

 do not always follow such a paradigm 

 (Peterson 1979). A series of tests of 

 this basic hypothesis has been carried out 

 in the Apalachicola Bay system over the 

 past 3 years. 



Inverse correlations between predator 

 and prey population do exist in the 

 Apalachicola estuary (Sheridan and 

 Livingston 1983). Macroinf aunal abundance 

 often declines precipitously during 

 periods of peak abundance of the chief 

 sciaenid predators (Mahoney and Livingston 

 1982). Such correlative results suggest 

 that fishes may have a direct influence on 

 the infaunal assemblages through 

 predation. In grassbed areas, however, 

 infaunal biomass is not affected because 

 larger species (burrowing deeper in the 

 sediments) are not influenced by such 

 predation. Also, recent experiments 

 indicate that macroinvertebrate 

 assemblages in East Bay remain largely 

 unaffected by predation pressure from 

 fishes in the late winter and spring and 

 by motile invertebrates (penaeid shrimp, 

 blue crabs) in the summer/fall (Mahoney 

 and Livingston 1982; Livingston unpubl.). 

 Thus, predation does not appear to play a 

 decisive role in the regulation of prey 

 density or macroinvertebrate community 

 structure in oligohaline portions of the 

 estuary during periods of peak predation 

 pressure. 



One possible explanation of the 

 apparent contradiction of the predation 

 paradigm could lie in the recruitment 

 potential of the dominant infaunal 

 species. In a series of experiments with 

 azoic sediments (i.e., devoid of 

 macroinvertebrates), Mahoney (1982) found 

 that infaunal larval recruitment was a 

 deciding factor in the population dynamics 



