Cyclic Trophic Relationships of Fishes in an Unpolluted, River-Dominated Estuary in North Florida

Abstract

Regular patterns in seasonal occurrence of dominant fishes were observed over a six-year period in the Apalachicola estuary of north Florida. Examination of potential physico-chemical and biological community determinants has led to the hypothesis that trophic relationships and underlying physical-biological inter-actions structure this estuarine fish community. Six species (Anchoa mitchilli, Micropogonias undulatus, Leiostomus xanthurus, Cynoscion arenarius, Brevoortia patronus, and Bairdiella chrysura) comprise 85% of the trawl-susceptible fishes in the Apalachicola system, and each is characterized by distinctive seasonal abundances and trophic spectra. Two benthic omnivores (Micropogonias and Leiostomus) exhibit high spatial and temporal overlap but differ in prey type and size. These two species utilize the estuary subsequent to high river discharge/detritus input and concurrent with maximum benthic standing crops. Two epibenthic carnivores (Cynoscion and Bairdiella) also use the estuary but differ in times of peak abundances and in prey types. Two planktivores (Brevoortia and Anchoa) also frequent the estuary but during different seasons (spring and fall, respectively), yet neither cooccurs with the maximum zooplankton standing crop (summer). Anchoa is prevented from doing so by the piscivorous Cynoscion population. Thus, regular seasonal progressions of dominant fishes are linked to available trophic resources, competition, and predation, which are in turn dependent upon such factors as river flow, detrital input, plankton production, and offshore processes. The data indicate that the trophic organization of the Apalachicola estuary is highly structured and is not simply the result of a series of physically forced events superimposed over a network of trophic opportunism.