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To examine the influence of river-borne organic material on estuarine communities, we conducted a dual stable isotope study in an estuary heavily influenced by alluvial runoff. Despite significant alluvial influence, secondary production in Apalachicola Bay depends more upon estuarine primary production than upon a detrital food web supported by floodplain primary production. Two simple mixing models, floodplain-marine and floodplain-estuarine, indicated that the upper limits for the contribution of terrestrial organic matter to estuarine consumer diets averaged 37, 25, and 27% and 20, 19, and 25% for East Bay, Cat Point, and Dry Bar, respectively. Systematic ␦ 13 C variation of consumer organisms was found for differing locations and attributed to increasing influence of terrestrial organic matter and 13 C-depleted dissolved inorganic carbon (DIC) closer to the river mouth. The ␦ 34 S data exhibited significant variation with river flow that was attributed to an admixture of terrestrial floodplain detritus with estuarine and marine organic matter. Both ␦ C and ␦ 34S isotope data demonstrated clear distinctions between benthic and water column feeding types. Our results suggest that the estuary is dependent on riverine inflows to provide floodplain detritus during the high-flow period and dissolved nutrients for estuarine primary productivity during the lowflow season. Any alteration of river hydrology may adversely affect estuarine secondary production, especially during the low-flow period when the estuary is dependent on input of dissolved nutrients to maintain a high level of primary productivity.Seminal works by Darnell (1958Darnell ( , 1961 and Odum and Heald (1972) focused on the role of detritus in the diets of a variety of fishes and macroinvertebrates. Yet, despite these and numerous other more recent feeding studies, the importance of organic detritus in providing nutritional support for estuarine food webs remains unclear. With the advent of stable isotope techniques, our understanding of the origins and flows of organic matter in estuarine, salt marsh, and nearshore environments has been greatly improved (Haines and

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