Concentration-dependent Stable Isotope Analysis of Consumers in the Upper Reaches of a Freshwater-dominated Estuary: Apalachicola Bay,FL, USA

The goals of this study were to quantify organic matter source utilization by consumers in the freshwater-dominated region (East Bay) of a high river flow estuary and compare the results to consumers in marine-influenced sites of the same estuary to understand how organic matter utilization by consumers may be changing along the salinity gradient. We used the results from these evaluations to establish the baseline against which we isotopically determined trophic level for consumers in East Bay. Average isotope values for consumers sampled in East Bay ranged from −20.1‰ to −24.8‰ for carbon and from 8.9‰ to 14.3‰ for sulfur. These values were well-constrained by the four identified sources: plankton, benthic organic matter, macroalgae, and terrestrial detritus. Application of a concentration-corrected mixing model resulted in contributions of benthic production and detrital sources (averaged over the food web) to East Bay consumers of 41% and 33%, respectively, with the remainder made up of plankton and benthic macroalage. While benthic organic matter was an important organic matter source for consumers at both sites, we found that the influence of terrestrial detritus varied significantly throughout the bay. Terrestrial detritus contributed only 18% of average total organic matter in organisms inhabiting marine-influenced sites. Although terrestrial detritus did contribute to all consumers examined, most fish species in Apalachicola Bay reflect a greater reliance on autochthonous sources. Our results suggest that, while terrestrial detritus does appear to be a major contributor to commercially important shellfish species (most notably oysters and penaeid shrimp), it is not the major source fueling the diversity of secondary production in Apalachicola Bay. Thus, production in Apalachicola Bay is highly dependent on riverine influx in two ways: (1) economically important bivalves and crustaceans are being fueled by terrestrial organic matter supplied by river flooding and (2) secondary and above consumer fish species are supported by in situ production which, in turn, is reliant on nutrients supplied by the Apalachicola River. These findings are significant in light of decisions regarding water usage and river flow restrictions in the Apalachicola-Chattahoochee-Flint drainage basin. The results of this study confirm that in situ estuarine organic matter is the dominant source supporting secondary production in this river-dominated estuary.