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 k₆₀₀ CH₄ (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.