The stable carbon isotope composition of dissolved inorganic carbon [DIC> was measured in Tampa Bay and Florida Bay. The dependence of isotopic composition was evaluated in terms of atmospheric CO2 exchange, carbon exchange between fresh water and seawater (i.e. salinity) and DIC derived from the reaction between calcium carbonate and organically derived CO2. The extent of organic carbon oxidation and the magnitude of organic carbon loading [pollution> in an estuary have implications for variations in the Δ13C of DIC and its use as an indicator of the relative amounts of land and marine derived organic and inorganic carbon in paleogeographic studies.¿ 1997 American Geophysical Union