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

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 (k(600)). This is the first study that directly compares k(600) of CO2, CH4, and N2O in an estuary with the aim to evaluate the accuracy of using a uniform k(600) value for estimating water-air fluxes. We calculated 155 k(600) 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 k(600) values showed a large range over the entire estuary (0.1-198.6 cm h(-1)) with slightly lower k(600) in the lower compared to the upper estuary. Overall, temporal variability was greater than spatial variability of k(600). We found the highest variability of k(600) between gas species in the lower estuary, whereas the variability was less distinct in the upper estuary. In the Maroochy estuary, k(600)CO(2) (mean 26.4 +/- 37.3 cm h(-1)) was mostly higher than k(600)CH(4) (mean 10.9 +/- 10.6 cm h(-1)) and k(600)N(2)O (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 k(600) models intended for CO2 may not accurately predict CH4 and N2O fluxes in estuaries. Our tested k(600) 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 k(600) of all three gases.

Automated summary

Spatial and temporal variability, as well as poor predictability of gas transfer velocity (k(600)), contribute to high global uncertainties in water-air CO2, CH4, and N2O fluxes from estuaries. The study in the Maroochy estuary found that temporal variability of k(600) was greater than spatial variability, with the highest variability of k(600) between gas species observed in the lower estuary. Additionally, k(600)CO(2) was mostly higher than k(600)CH(4) and k(600)N(2)O, likely due to chemical and enzymatic enhancements and/or microbial activity in the surface microlayer.