On Factors Influencing Air‐Water Gas Exchange in Emergent Wetlands

Abstract

Knowledge of gas exchange in wetlands is important in order to determine fluxes of climatically and biogeochemically important trace gases and to conduct mass balances for metabolism studies. Very few studies have been conducted to quantify gas transfer velocities in wetlands, and many wind speed/gas exchange parameterizations used in oceanographic or limnological settings are inappropriate under conditions found in wetlands. Here six measurements of gas transfer velocities are made with SF6 tracer release experiments in three different years in the Everglades, a subtropical peatland with surface water flowing through emergent vegetation. The experiments were conducted under different flow conditions and with different amounts of emergent vegetation to determine the influence of wind, rain, water flow, waterside thermal convection, and vegetation on air‐water gas exchange in wetlands. Measured gas transfer velocities under the different conditions ranged from 1.1 cm h−1 during baseline conditions to 3.2 cm h−1 when rain and water flow rates were high. Commonly used wind speed/gas exchange relationships would overestimate the gas transfer velocity by a factor of 1.2 to 6.8. Gas exchange due to thermal convection was relatively constant and accounted for 14 to 51% of the total measured gas exchange. Differences in rain and water flow among the different years were responsible for the variability in gas exchange, with flow accounting for 37 to 77% of the gas exchange, and rain responsible for up to 40%.

Wetland methane emissions dominated by plant‐mediated fluxes: Contrasting emissions pathways and seasons within a shallow freshwater subtropical wetland

“…Higher night‐time fluxes may also occur due to convective overturning of the water column (Poindexter et al ). Our estimated convection rate (13.6%) was within range of wetland convection studies of Ho et al (), who found that convection contributed from 14% to 50% of the measured gas exchange in shallow sloughs within the Everglades, U.S.A., and Poindexter et al (), who showed convection was responsible for 32% of annual wetland emissions from a marsh in northern California, U.S.A. Our somewhat lower convection estimates may be related to the high coverage of lilies during C1 reducing the magnitude of the net solar radiation daily balance.…”

“…To estimate the relative importance of night‐time convection as a wetland CH 4 transport pathway, we utilized time series temperature data from C1 to estimate the convection gas transfer velocity ( k 600convection ) with the equation from Ho et al (): where q is the net heat flux, T is the temperature between 6°C and 40°C, and Pr is the Prandtl number at T . By then applying k 600convection to our time series CH 4 data, we could then estimate the night‐time air–water CH 4 convection driven fluxes and determine the proportionate role of convection to our average C1 night‐time chamber flux measurements.…”

Turbulence and Gas Transfer Velocities in Sheltered Flooded Forests of the Amazon Basin

“…Measured ε and buoyancy flux predicted from hourly temperature change, β T , were similar under cooling (Figure 3). This similarity implies that ε and k 600 can be computed for other sites from temperature change when β < 0, as computed previously for shallow herbaceous wetlands (Ho et al, 2018;Poindexter et al, 2016;Poindexter & Variano, 2013). The lower measured values of ε during heating under calm conditions are to be expected as the depth of the measurement volume of the ADV was within the stratified diurnal thermocline rather than within an actively mixing layer.…”

“…Flux, F = k (C w -C eq ), where k is gas transfer velocity, C w is concentration in the water, and C eq is concentration in the water at equilibrium with the atmosphere. In the SRM, k 600 = c 1 (ε·ν) 1/4 ·Sc −n where k 600 is the gas transfer coefficient normalized to CO 2 at 20°C, ε is rate of dissipation of turbulent kinetic energy, ν is kinematic viscosity, Sc is the Schmidt number, n is −1/2 with variability dependent on extent of surface films (Katul et al, 2018) but see Ho et al (2018), and coefficient c 1 is~0.5 (Katul et al, 2018;Lamont & Scott, 1970;Zappa et al, 2007) (See SI S.1.10). Rates of dissipation of turbulent kinetic energy can be obtained by direct measurements (Gålfalk et al, 2013;MacIntyre et al, 2018;Tedford et al, 2014).…”

Parameterizing Air‐Water Gas Exchange in the Shallow, Microtidal New River Estuary

“…However, it remains likely that factors other than wind-or convection-driven turbulence affected k 600 and may have played a role in increased nighttime k 600 . One important such factor that was not accounted for in this study was bottom-driven turbulence, which has been shown to at times significantly enhance gas transfer velocity, particularly in shallow estuaries (Ho et al, 2016(Ho et al, , 2018Maurice et al, 2017;Raymond & Cole, 2001;Rosentreter et al, 2017;Zappa et al, 2007). Because the NRE is quite shallow (average depth <2 m), it is plausible that bottom-driven turbulence may have played an important role in enhancing k. While measurements related to water turbulence or velocity were not possible during the present study, such an investigation into bottom-driven turbulence remains an important avenue for future research.…”