Flux and bioavailability of Cape Fear River and rainwater dissolved organic carbon to Long Bay, southeastern United States

This study presents the first comparison of riverine and rainwater dissolved organic carbon (DOC) flux to a coastal region, and the first examination of rainwater DOC bioavailability. The bioavailability of DOC from these two sources was determined because its impact on coastal waters depends on whether DOC is labile or refractory. The study site, Long Bay, off the North Carolina and South Carolina coasts in the southeastern United States, receives external sources of DOC from the Cape Fear River system and rainwater. Conservative mixing of estuarine DOC, and a consistent DOC value for the river sources entering the Cape Fear Estuary (Cape Fear and Black rivers combined: 937+/-178 muM, n=12; Northeast Cape Fear River: 1266+/-196 muM, n=8) allowed for a calculation of DOC flux using river DOC values and U. S. G. S. river flow data. Rainwater DOC flux was determined by a deposition calculation at a rainwater collection station in Wilmington, N.C., 40 km from Long Bay. Riverine DOC flux was determined to be (77x10(9) gCyr(-1)) or roughly 4x the rainwater flux (21x10(9) gCyr(-1)) on an annual basis. The bioavailability of the riverine DOC (9.0+/-4.5%) was much less than that of rainwater (63+/-14%). Therefore, taking into account only bioavailable DOC, rainwater provides more labile DOC (13x10(9) gCyr(-1)) to Long Bay than riverwater (7x10(9) gCyr(-1)) meaning that rainwater is important for fueling secondary productivity whereas river DOC is important for long-term storage of carbon.