Examination of coupling between primary and secondary production in a river‐dominated estuary: Apalachicola Bay, Florida, U.S.A.

Abstract

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 δ13C variation of consumer organisms was found for differing locations and attributed to increasing influence of terrestrial organic matter and 13C‐depleted dissolved inorganic carbon (DIC) closer to the river mouth. The δ34S data exhibited significant variation with river flow that was attributed to an admixture of terrestrial floodplain detritus with estuarine and marine organic matter. Both δ13C and δ34S 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 low‐flow 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.