Oxygen and carbon dioxide mass balance for the estuarine‐intertidal marsh complex of five rivers in the southeastern U.S.

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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 m−3 d−1 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 m−2 d−1 and 50–120 mmol m−2 d−1 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.