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 ¿M, n = 12; Northeast Cape Fear River: 1266 ¿ 196 ¿M, 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 (77 ¿ 109 gCyr-1) or roughly 4x the rainwater flux (21 ¿ 109 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 (13 ¿ 109 gCyr-1) to Long Bay than riverwater (7 ¿ 109 gCyr-1) meaning that rainwater is important for fueling secondary productivity whereas river DOC is important for long-term storage of carbon.