Limnol. Oceanogr., 44(3), 1999, 639–649

We measured dissolved O2 concentrations, pCO2 values, and respiratory rates in five estuaries of the southeastern U.S. in October 1995 and July 1996. In the low-salinity sections of the coastal plain rivers, dissolved O2 saturation states were typically only 50%, while pCO2 values were over 4,000 matm. Respiratory rates measured concurrently in estuarine water averaged 8 and 23 mmol m23 d21 in October 1995 and July 1996, but they showed little variability either within or among the five estuaries. Benthic chamber incubations in the adjacent intertidal marshes indicated fluxes of 30–40 mmol m22 d21 and 50–120 mmol m22 d21 for O2 and total dissolved inorganic carbon (DIC), respectively. For the Satilla River estuary, simple calculations revealed that neither respiratory activity in estuarine waters and sediments nor any other within-estuary process (not including the intertidal marsh system) was sufficient to account for the observed O2 concentrations and pCO2 values. Dissolved oxygen concentrations in four other southeastern U.S. estuaries fit the same general pattern as the Satilla, and likewise, within-estuary processes could not explain observed gas concentrations. Measured O2 concentrations, pCO2 values, pelagic respiratory rates, and benthic fluxes were used to construct a mass-balance model, focusing on the influence of the extensive intertidal marshes on O2 and CO2 mass balance and water–atmosphere gas exchange in the five estuaries. Results indicate that respiratory activity in the sediments and overlying water of the marshes during high tide leaves a signal that is funneled back to the estuary during ebb tide and can account for the estuarine gas concentrations and fluxes. Both experimental and modeling approaches argue that the intertidal marshes of the southeastern U.S. export considerable amounts of inorganic respiratory products to the estuaries and that ‘‘outwelling’’ of organic matter to the estuaries is a minor process by comparison. The DIC exported to the coastal ocean, however, is only a small fraction of the total gas mass flow between the marsh, estuary, and atmosphere. Early recognition of coastal intertidal marshes as highly productive ecosystems led to the hypothesis that marshes contribute significant amounts of organic carbon to estuaries and the coastal ocean (Teal 1962; Odum 1968, 1980). This hypothesis, based primarily on the coastal marshes of Georgia, was further supported by observations of estuarine oxygen deficits (Ragotzkie 1959; Pomeroy et al. 1972; Frankenberg 1975; Frankenberg and Shimmel 1975). The outwelling hypothesis was later challenged, at first because there were insufficient supporting data (Nixon 1980) and later because new data and more detailed models of estuaries did not reveal large fluxes of organic matter from marshes (Woodwell et al. 1977; Wiegert et al. 1981; Chalmers et al. 1985; Childers et al. 1993). For example, the model of Wiegert et al. (1981) suggested a net export of about 25% of