Gas transfer velocities for quantifying methane, oxygen and other gas fluxes through the air-water interface of wetlands with emergent vegetation

Empirical models for the gas transfer velocity, k, in the ocean, lakes and rivers are fairly well established, but there are few data to predict k for wetlands. We have conducted experiments in a simulated emergent marsh in the laboratory to explore the relationship between k, wind shear and thermal convection. Now we identify the implications of these results for gas transfer in actual wetlands by (1) quantifying the range of wind conditions in emergent vegetation canopies and the range of thermal convection intensities in wetland water columns, and (2) describing the non-linear interaction of these two stirring forces over their relevant ranges in wetlands. We measured mean wind speeds and wind speed variance within the shearless region of a Schoenoplectus-Typha marsh canopy in the Sacramento-San Joaquin Delta (Northern California, USA). The mean wind speed within this region, <Ucanopy>, is significantly smaller than wind above the canopy. Based on our laboratory experiments, for calm or even average wind conditions in this emergent marsh k600 is only on the order 0.1 cm hr-1 (for neutrally or stably stratified water columns). We parameterize unstable thermal stratification and the resulting thermal convection using the heat flux through the air-water interface, q. We analyzed a water temperature record for the Schoenoplectus-Typha marsh to obtain a long-term heat flux record. We used these heat flux data along with short-term heat flux data from other wetlands in the literature to identify the range of the gas transfer velocity associated with thermal convection in wetlands. The typical range of heat fluxes through water columns shaded by closed emergent canopies (-200 W m-2 to +200 W m-2) yields k600 values of 0.5 - 2.5 cm hr-1 according to the model we developed in the laboratory. Thus for calm or average wind conditions, the gas transfer velocity associated with thermal convection is significantly larger than the gas transfer velocity associated with wind shear. Because of the diurnal pattern in water column heat flux that follows the diurnal pattern in incoming solar radiation, this difference means gas transfer velocities are expected to vary diurnally during calm or average wind conditions, peaking late at night and early in the morning. Conversely for very windy conditions, <Ucanopy> alone may determine the gas transfer velocity even when high heat fluxes out of the water column are relatively high. For the calculation of k600 from <Ucanopy>, we developed an enhancement factor to account for the very high wind speed variance observed in the Schoenoplectus-Typha emergent canopy and likely seen in other emergent canopies as well. These wetland targeted gas transfer velocities will improve the accuracy of wetland gas flux measurements and models and enable the partitioning of net gas fluxes from wetlands into plant-mediated fluxes, ebullitive fluxes and fluxes due to the hydrodynamic transport of dissolved gases through the water column.