Variability of the gas transfer velocity of CO2 in a macrotidal estuary (the Scheldt)

We report a large set of 295 interfacial carbon dioxide (CO2) flux measurements obtained in the Scheldt estuary in November 2002 and April 2003, using the floating chamber method. From concomitant measurements of the air-water CO2 gradient, we computed the gas transfer velocity of CO2. The gas transfer velocity is well correlated to wind speed and a simple linear regression function gives the most consistent fit to the data. Based on water current measurements, we estimated the contribution of water current induced turbulence to the gas transfer velocity, using the conceptual relationship of O'Connor and Dobbins (1958). This allowed us to construct an empirical relationship to compute the gas transfer velocity of CO2 that accounts for the contribution of wind and waiter current. Based on this relationship, the spatial and temporal variability of the gas transfer velocity in the Scheldt estuary was investigated. Water currents contribute significantly to the gas transfer velocity, but the spatial and temporal variability (from daily to seasonal scales) is mainly related to wind speed variability.