Spatial and temporal variability of CO2and CH4gas transfer velocities and quantification of the CH4microbubble flux in mangrove dominated estuaries

Abstract

AbstractGas transfer velocities (k) of CO2and CH4were 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 specifick.k600‐CO2andk600‐CH4(knormalized to the Schmidt number of 600) varied greatly within and between mangrove creeks, ranging from 0.9 cm h−1to 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 controllingk. An empirical relationship that accounts for current velocity and a linearly additive contribution of wind speed and water depth was a good predictor ofk600‐CO2(R2 = 0.67) andk600‐CH4(R2 = 0.57) in the mangrove creeks in Australia. In a side‐by‐side study, good agreement was found betweenkdetermined from this new floating chamber and a3He/SF6dual tracer release experiment (∼5% discrepancy).k600‐CH4correlated well withk600‐CO2(R2 = 0.81), however,k600‐CH4was on average 1.2 times higher thank600‐CO2, most likely reflecting a microbubble flux contribution. The microbubble flux contributed up to 73% of the total CH4flux and was best predicted by a model that included CH4supersaturation, temperature, and current velocity. A large overestimation was found for both CO2and CH4fluxes when calculated using empirically derivedkmodels from previous studies in estuaries. The high temporal and spatial variabilities ofkCO2andkCH4highlights the importance of site specific transfer velocity measurements in dynamic ecosystems such as mangrove estuaries.