The cycling of dissolved inorganic carbon (DIC) and the role of tidal marshes in estuarine DIC dynamics were studied in a Virginia tidal freshwater marsh and adjacent estuary. DIC was measured over diurnal cycles in different seasons in a marsh tidal creek and at the junction of the creek with the adjacent Pamunkey River. In the creek, DIC concentrations around high tide were controlled by the same processes affecting whole-estuary DIC gradients. Near low tide, DIC concentrations were 1.5-5-fold enriched relative to high tide concentrations, indicating an input of DIC from the marsh. Similar patterns (although dampened in magnitude) were observed at the creek mouth and indicated that DIC was exported from the marsh. Marsh pore-water DIC concentrations were up to 5 mmol L Ϫ1 greater than those in the creek and suggested a significant input of sediment pore water to the creek. A model of tidal marsh DIC export showed that, on a seasonal basis, DIC export rates were influenced by water temperature. The composition of exported DIC averaged 19% dissolved CO 2 and 81% HCO and CO . Although CO 2 can belost to the atmosphere during transit through the estuary, DIC in the form of carbonate alkalinity is subject to export from the estuary to the coastal ocean. When extrapolated to an estuarywide scale, the export of marsh-derived DIC to the York River estuary explained a significant portion (47 Ϯ 23%) of excess DIC production (i.e., DIC in excess of that expected from conservative mixing between seawater and freshwater and equilibrium with the atmosphere) in this system. Therefore, CO 2 supersaturation, by itself, does not indicate that an estuary is net heterotrophic.One approach to understanding the cycling of organic carbon within ecosystems is through measurements of total system metabolism, given that the production and removal of organic matter are intimately linked to total dissolved inorganic carbon (DIC, or ⌺CO 2 ) and O 2 cycling. Recent studies of estuarine CO 2 and DIC dynamics have shown that estuaries are generally supersaturated with respect to CO 2 and exhibit high rates of net heterotrophy (i.e., respiration Ͼ photosynthesis; Smith and Hollibaugh 1993;Frankignoulle et al. 1998; Gatusso et al. 1998), although the main stem of Chesapeake Bay is net autotrophic (Kemp et al. 1997). Sources of CO 2 and DIC to estuarine waters include watercolumn and benthic respiration, riverine and groundwater inputs, photodegradation of dissolved organic matter, and inputs from intertidal marshes (Hopkinson and Vallino 1995;Kemp et al. 1997;Cai and Wang 1998). Accurate quantification of rates of net heterotrophy requires that one account 1 Corresponding author. Present address:
