Divergent Gas Transfer Velocities of CO2, CH4, and N2O Over Spatial and Temporal Gradients in a Subtropical Estuary

Abstract

High global uncertainties remain in water-air CO2, CH4, and N2O fluxes from estuaries due to spatial and temporal variability and the poor predictability of the gas transfer velocity (k600). This is the first study that directly compares k600 of CO2, CH4, and N2O in an estuary with the aim to evaluate the accuracy of using a uniform k600 value for estimating water-air fluxes. We calculated 155 k600 values from CO2, CH4, and N2O fluxes over spatial (across, along) and temporal (tidal cycle) surveys in the subtropical Maroochy estuary using the floating chamber method. Combined k600 values showed a large range over the entire estuary (0.1–198.6 cm h−1) with slightly lower k600 in the lower compared to the upper estuary. Overall, temporal variability was greater than spatial variability of k600. We found the highest variability of k600 between gas species in the lower estuary, whereas the variability was less distinct in the upper estuary. In the Maroochy estuary, k600CO2 (mean 26.4 ± 37.3 cm h−1) was mostly higher than k600CH4 (mean 10.9 ± 10.6 cm h−1) and k600N2O (mean 9.9 ± 12.3 cm h−1), likely due to chemical and enzymatic enhancements and/or microbial activity in the surface microlayer. We demonstrate that empirical k600 models intended for CO2 may not accurately predict CH4 and N2O fluxes in estuaries. Our tested k600 models predicted the measured fluxes within an uncertainty range of 5%–40% (over or underestimation), but precise flux estimates should be based on in situ k600 of all three gases.