Plankton and dissolved inorganic carbon isotopic composition in a river-dominated estuary: Apalachicola Bay, Florida

To characterize the isotopic composition of organisms at the base of the food web and the controls on their variability, the concentration and delta(13)C isotopic composition of dissolved inorganic carbon (DIC) and plankton delta(13)C, delta(15)N, and delta(34)S were measured. The measurements were made during periods of high and low river flow in Apalachicola Bay, Florida, United States, otter 3 yr. DIC concentration and delta(13)C values were related to salinity, indicating that conservative mixing of riverine and marine waters was responsible for the overall distributions. The usefulness of DIC delta(13)C data for characterizing the trophic processes within the estuary was dependent upon the residence time of water within the estuary. Despite the input of large quantities of terrestrial organic mater, net heterotrophy was indicated in only one fall season. Plankton delta(13)C values varied from -22 parts per thousand to -30 parts per thousand and were directly related to estuarine DIC delta(13)C, offset by a factor of roughly -20 parts per thousand. This offset factor varied with salinity. Values of delta(34)S in estuarine plankton (station means ranged from 11.4 parts per thousand to 13.1 parts per thousand) were depleted relative to marine plankton (17.7 +/- 0.4 parts per thousand) possibly due to the admixture of S-34-depleted sedimentary sulfide with estuarine samples. Values of delta(34)S in plankton were not related to delta(13)C values of plankton and were only weakly correlated to the salinity of the water from which the plankton were collected, indicating that marine sulfate was the primary source of planktonic sulfur. Values of delta(15)N in plankton varied from 5.5 parts per thousand to 10.7 parts per thousand and appeared related to dominance of the sample by phytoplankton or zooplankton. Estuarine plankton was N-15 enriched relative to offshore plankton and estuarine sediment.