Spatial and temporal variability of CO 2 and CH 4 gas transfer velocities and quantification of the CH 4 microbubble flux in mangrove dominated estuaries

Abstract

Gas transfer velocities ( k ) of CO 2 and CH 4 were determined from 209 deployments of a newly designed floating chamber in six mangrove dominated estuaries in Australia and the United States to estimate mangrove system specific k . k 600 ‐ CO 2 and k 600 ‐ CH 4 ( k normalized to the Schmidt number of 600) varied greatly within and between mangrove creeks, ranging from 0.9 cm h −1 to 28.3 cm h −1 . The gas transfer velocity correlated well with current velocity at all study sites suggesting current generated turbulence was the main driver controlling k . An empirical relationship that accounts for current velocity and a linearly additive contribution of wind speed and water depth was a good predictor of k 600 ‐CO 2 ( R 2 = 0.67) and k 600 ‐CH 4 ( R 2 = 0.57) in the mangrove creeks in Australia. In a side‐by‐side study, good agreement was found between k determined from this new floating chamber and a 3 He/SF 6 dual tracer release experiment (∼5% discrepancy). k 600 ‐ CH 4 correlated well with k 600 ‐ CO 2 ( R 2 = 0.81), however, k 600 ‐ CH 4 was on average 1.2 times higher than k 600 ‐ CO 2 , most likely reflecting a microbubble flux contribution. The microbubble flux contributed up to 73% of the total CH 4 flux and was best predicted by a model that included CH 4 supersaturation, temperature, and current velocity. A large overestimation was found for both CO 2 and CH 4 fluxes when calculated using empirically derived k models from previous studies in estuaries. The high temporal and spatial variabilities of k CO 2 and k CH 4 highlights the importance of site specific transfer velocity measurements in dynamic ecosystems such as mangrove estuaries.

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