Divergent Gas Transfer Velocities of CO<sub>2</sub>, CH<sub>4</sub>, and N<sub>2</sub>O Over Spatial and Temporal Gradients in a Subtropical Estuary
Judith A. Rosentreter, Naomi S. Wells, Amber J. Ulseth, Bradley D. Eyre
Abstract
Abstract High global uncertainties remain in water‐air CO 2 , CH 4 , and N 2 O 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 CO 2 , CH 4 , and N 2 O 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 CO 2 , CH 4 , and N 2 O 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 CO 2 may not accurately predict CH 4 and N 2 O 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.