Strong Subseasonal\nVariability of Oxic Methane Production\nChallenges Methane Budgeting in Freshwater Lakes

Methane (CH<sub>4</sub>) accumulation in the well-oxygenated\nlake\nepilimnion enhances the diffusive atmospheric CH<sub>4</sub> emission.\nBoth lateral transport and in situ oxic methane production (OMP) have\nbeen suggested as potential sources. While the latter has been recently\nsupported by increasing evidence, quantifying the exact contribution\nof OMP to atmospheric emissions remains challenging. Based on a large\nhigh-resolution field data set collected during 2019–2020 in\nthe deep stratified Lake Stechlin and on three-dimensional hydrodynamic\nmodeling, we improved existing CH<sub>4</sub> budgets by resolving\neach component of the mass balance model at a seasonal scale and therefore\nbetter constrained the residual OMP. All terms in our model showed\na large temporal variability at scales from intraday to seasonal,\nand the modeled OMP was most sensitive to the surface CH<sub>4</sub> flux estimates. Future efforts are needed to reduce the uncertainties\nin estimating OMP rates using the mass balance approach by increasing\nthe frequency of atmospheric CH<sub>4</sub> flux measurements.