Windy Wetlands: The Movement of Emergent Vegetation Facilitates Gas Transfer

Wind-driven movement of emergent vegetation has been an overlooked mixing-mechanism in wetland gas models. To estimate the occurrence this movement in natural systems and to better understand the mechanics of vegetation movement, in situ videos were taken at the Twitchell Island wetlands in the Sacramento-San Joaquin Delta. To understand the magnitude of this effect on gas transfer, a laboratory experiment was conducted. An array of plastic tubes, representing vegetation, was mechanically forced at a range of frequencies and amplitudes matching those observed in the field. Starting from deoxygenated water, we measured dissolved oxygen (DO) at mid-depth in the water column as the water re-equilibrated with the atmosphere. From this DO timeseries, we calculated the gas transfer velocity, k, using the thin film gas transport model. This measurement of k was independent of the water-wall interactions, which can be significant in laboratory-scale tanks. Our experiments have shown that the movement of vegetation stirs the water column and thus contributes to the transfer of dissolved gases across the air-water interface. With small amplitude motions (0.5 cm), the gas transfer velocity was independent of frequency, while increased transfer was observed with movements of higher frequency (>0.5 Hz) and amplitude (>0.8 cm). These results can help improve current wetland gas budgets and predictions of biogeochemical processes.