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We investigated the seasonal changes and the interactions of the CO₂ system in a marsh‐dominated estuary, the Duplin River, on Sapelo Island, Georgia. Surface water pCO₂, total alkalinity (TA), and total dissolved inorganic carbon (DIC) showed a pronounced seasonal progression in this system. The estuary released 256‐306 g C m^‐2 (of water) yr^‐1 of CO₂ to the atmosphere and exported 109 g C m^‐2 (of water) yr^‐1 of DIC to adjacent coastal waters. There was a clear seasonal pattern in both fluxes with spring minima, and late summer and fall maxima. Release of inorganic carbon as a respiratory product from surrounding salt marshes is the primary process supporting these two carbon export terms. Concurrently, TA was exported from the Duplin River mainly in summer and fall as a result of anaerobic respiration in marshes. Based on data from the upper Duplin River, export of inorganic carbon from salt …