Gas Exchange Rates in the Parker River Estuary, Massachusetts.

Gas exchange rates between natural waters and the atmosphere are an important component of our understanding of the dynamics of biologically active gases. For example, estimates of whole system metabolism must account for the transfer of gas (OJ between the atmosphere and water. Previous studies in estuaries have relied on dome measurements to estimate O2 exchange rates ( 1,2). In this study we measured gas exchange rates using sulfur hexafluoride (SF,) as a tracer. SF6 is well suited for this application because it is chemically and biologically nonreactive, occurs at low background levels, and extremely low concentrations are readily detectable (3). The gas exchange coefficient, k, calculated for SF, can be related to the exchange coefficient for other gases (4). We injected -0.004 moles of SF, into the Parker River estuary, Newbury Massachusetts, and monitored the evasion of the gas over time by the decrease in its total mass. The tracer was allowed to mix for one tidal cycle and we then sampled SF, concentration in the water at each of seven successive high tides. Surface water samples were drawn into loo-ml glass syringes, transported, submerged in river water, to a field laboratory, and analyzed within 6 h using gas chromatography with electron capture detection. Windspeed and precipitation data were recorded continuously. Several previous surveys were used to determine cross-sectional reas along a IO-km stretch of the estuary. The total mass of SF, in the estuary was calculated by integrating concentration and water volume by estuarine distance. The distribution of the tracer changed over time in relation to processes controlling mixing and loss to the atmosphere (Fig. la). After the initial tidal cycle the tracer plume measured 5.2 km in length, and the distribution was gaussian. The exchange coefficient, k, and mass are related by the function k = In (M/Mo)h/t where M is the measured mass of SF, in the estuary, M. is the previously measured mass of SFs, h is the depth, and t is the time between samplings. Calculated values for kSF6 range from 1.1-6.2 cm. h-‘. Fluctuations in k are well correlated with wind velocity [in agreement with previous studies (4, 5)] and with precipitation (Fig. 1 b). These values are lower than those predicted from wind relations established from dome studies (Fig. 1 c). For estuarine systems with complicated geometry (e.g., channel longitudinal direction, marsh grass, and high tidal range), direct measurement of SF, evasion may be a more accurate determination of gas exchange rates. The importance of gas exchange as a process influencing the determination of system metabolism was determined by applying our measured gas exchange coefficient for SF6 to the calculation of O2 gas exchange. System respiration was calculated by mass loss of dissolved oxygen between dusk and dawn (Fig. Id) and corrected for gas exchange with the atmosphere. The gas transfer velocities of O2 and SF6 are related by the function k,,