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

Abstract

The goals of this study were to quantify organic matter source utilization by consumers in the freshwaterdominated 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 wellconstrained 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.

An exploratory investigation of the landscape-lake interface: Land cover controls over consumer N and C isotopic composition in Lake Michigan rivermouths

“…Consumer δ has been observed from river to ocean in estuaries as well (Middelburg and Nieuwenhuize, 1998;Wilson et al, 2009Wilson et al, , 2010. More depleted δ 15 N isotopic composition of consumers in the nearshore of Lake Michigan relative to upstream sites is consistent with previous observations of Lake Michigan incorporating atmospheric N into the food web (MacGregor et al, 2001).…”

Fish trophic structure in estuaries, with particular emphasis on estuarine typology and zoogeography

“…In river‐dominated estuaries, fluvial input is an important source of detritus (Lin et al , 2007; Wilson et al , 2010). In some estuaries, particularly those that are deprived of an adequate river flow, detritus is mainly autochthonous and is usually derived from intertidal and subtidal plants; in the warm‐temperate Kromme Estuary, for example, Heymans & Baird (1995) determined that marsh halophytes were responsible for 78% of primary production, of which 90% was first broken down to detritus before entering the food web.…”

Fresh Water Inflow and Oyster Productivity in Apalachicola Bay, FL (USA)

“…Comparison of copepods and oysters suggests that ingestion of microzooplankton is less important for oysters and/or that oysters may be assimilating some depleted organic matter from the surface sediments or terrestrial particulate organic matter (POM). Wilson et al (2010) have suggested that benthic microalgae growing on the sediment surface may also be a significant food source to bay consumers.…”

“…The low δ 15 N values coupled with the understanding that benthic resuspension of surficial materials is likely common in this shallow, wind mixed system suggest that sediment POM contributes to oyster diets. In fact, Wilson et al (2010) estimated that benthic organic matter comprised about 20% of oyster diets. Chanton and Lewis (2002) also used δ 13 C values in a simple two endmember mixing model to estimate that about 40% of the organic matter in the sediments was of terrestrial origin.…”

“…The focus of this study was on potential links between the floodplain and bay consumers and it estimated that terrestrial detritus could contribute up to 38% of oyster diets (Chanton and Lewis 2002). The extensive data set developed by Chanton and Lewis in the early 1990s has recently been reexamined to assess the trophic position of various dominant bay species (Wilson et al 2009(Wilson et al , 2010, and confirmed oysters as primary consumers in the bay food web.…”