Fig. 1: The contribution of net oxic methane production to the diffusive CH₄ emission from lakes.

The contribution of net oxic methane production to the diffusive CH₄ emission, NOMC, was calculated in the different lakes from the surface CH₄ flux, F_surf, and the methane flux from the sediments, F_sed, obtained from different data sources: Lake Hallwil (Supplementary Table 1): F_surf from the “Hallwil relationship” that is based on the chamber measurements in Lake Hallwil⁶. F_sed, from the CH₄ pore water concentrations in the sediment core collected at 3 m water depth (F_sed = 2.8 mmol m⁻² day⁻¹, Supplementary Table 1). Lake Stechlin (Supplementary Table 3): Lower and upper limits of F_sed (F_sed = 1.8 mmol m⁻² day⁻¹ and F_sed = 2.0 mmol m⁻² day⁻¹) from the re-evaluation of the mesocosm experiments (Supplementary Table 2) providing upper and lower limit of NOMC, respectively (Supplementary Table 3). South Basin (average 2014, 2016): F_surf from the “Stechlin relationship”; North Basin (2016, a): F_surf from chamber measurements; North Basin (2016, b) F_surf from chamber measurements combined with the “Stechlin relationship” for the 20 June; Lake Stechlin South Basin (2017) (Supplementary Table 4): F_sed derived from CH₄ pore water measured in a single sediment core by ref. ¹¹, considering the CH₄ gradient in the top 2 cm and at 5 cm depth (F_sed = 0.08 mmol m⁻² day⁻¹ and F_sed = 0.26 mmol m⁻² day⁻¹) providing upper and lower limit of NOMC, respectively; F_surf from specific wind model of ref. ¹¹; Lake Cromwell: Data from ref. ⁵. Additional Lakes (Supplementary Table 5): based on the analysis of ref. ⁷ (see Supplementary Note 4). The ratio between the area of the sediment A_sed and the volume V_SML in the surface mixed layer SML, A_sed/V_SML, was estimated assuming a slope angle of 5° for the lake bed (Supplementary Note 4). The sensitivity to the slope angle is illustrated in Supplementary Fig. 5 assuming a slope angle of 3°.