Oxic methanogenesis is only a minor source of lake-wide diffusive CH4 emissions from lakes

In Lake Hallwil, the contribution of oxic methanogenesis to overall diffusive CH₄ emissions has been estimated to be 90%⁶ or 63–83%⁵, but we show here that NOMC ~ 17%.

In the mass balance of the SML extending from 0 to 5 m water depth^5,6, Günthel et al.⁵ used an average sediment flux of F_sed = 1.75 mmol m⁻² day⁻¹, averaging flux estimates of Donis et al.⁶ from two sediment cores, one collected at 3 m and the other at 7 m water depth. The δ¹³C of the CH₄ in the pore water of these two cores differ substantially⁶, indicating differences in production and oxidation of CH₄ between the sediments in the SML and at 7 m water depth. The estimate of F_sed in the SML should therefore be based on the core collected at 3 m water depth. Using the approach of Donis et al.⁶, the correct F_sed derived from the data of this core is F_sed = 2.8 mmol m⁻² day⁻¹ (Peeters et al.⁹, see Supplementary Note 2.1 for details).

Günthel et al.⁵ and Donis et al.⁶ apparently have erroneously used gas transfer coefficients instead of proper CH₄ fluxes to calculate emissions. This conclusion is demonstrated by the perfect agreement between the values published erroneously as CH₄ fluxes, F_surf, by Günthel et al.⁵ and the values of the gas transfer coefficients of CH₄ at 20 °C, k_CH4, calculated by us (Table 1). The values published by Donis et al.⁶ as CH₄ fluxes are very similar to these k_CH4 and therefore also do not represent CH₄ fluxes but gas transfer coefficients (for details, see Supplementary Note 2.2).

The gas transfer coefficient of CH₄ must be multiplied by the difference between the surface concentration (0.3 mmol m⁻³, ref. ⁶) and the atmospheric equilibrium concentration of CH₄ (CH_4,equ = 0.003 mmol m⁻³ at 20 °C⁹), i.e. by ~0.3 mmol m⁻³, to obtain F_surf. F_surf is therefore ~3.3 times smaller than the values of the gas transfer coefficients erroneously taken by Günthel et al.⁵ and Donis et al.⁶ as CH₄ fluxes (Table 1 and details in Supplementary Note 2.2).

Donis et al.⁶ and Günthel et al.⁵ used values obtained from measurements with floating chambers to calculate emissions, but these values claimed to represent F_surf appear to be in fact values for transfer coefficients, suggesting the same mistake as in the case of the wind models. Donis et al.⁶ stated: “Average flux (April–August 2016) is equal to 0.8 ± 0.2 mmol m⁻² d⁻¹ from MacIntyre relationship for positive buoyancy and to 0.6 ± 0.3 mmol m⁻² d⁻¹ from chamber measurements. The latter, not significantly different from the wind-based relationship, was used for the mass balance”. Günthel et al.⁵, co-authored by D. Donis, claim that the “MacIntyre relationship for positive buoyancy”¹⁰ provides an average value of 0.7 for F_surf, but in fact 0.7 is the average value for k_CH4 in unit m day⁻¹ (0.7 m d⁻¹, see Table 1) and F_surf for this model is 3.3 times smaller (0.21 mmol m⁻² d⁻¹, see Table 1). The value by Donis et al.⁶ for the MacIntyre relationship¹⁰ is even slightly larger than 0.7 and therefore clearly incompatible with F_surf but is rather a gas transfer coefficient as is obvious in the case of Günthel et al.⁵. The good agreement between the value for the gas transfer coefficient obtained from the MacIntyre model for positive buoyancy flux¹⁰ and the values from the chamber measurements suggests that the values from the chamber measurements are not gas fluxes but also gas transfer coefficients (see Supplementary Note 2.2 for more details).

Donis et al.⁶ derived from their chamber measurements the wind-based model “Hallwil relationship” specifically for Lake Hallwil. The establishment of this Hallwil relationship required that Donis et al.⁶ calculated gas transfer coefficients from their chamber measurements. In their Supplementary Fig. 4, Donis et al.⁶ show that the values from their chamber measurements agree well with those from the Hallwil relationship (Supplementary Fig. 2 and Supplementary Note 2.2). Note, however, that the values for the Hallwil relationship are in fact gas transfer coefficients and not F_surf, supporting that also the values from the chamber measurements represent gas transfer coefficients and not F_surf (Supplementary Fig. 2 and Supplementary Note 2.2 for more details). This conclusion implies that the values from the chamber measurements by Donis et al.⁶ must be multiplied by ~0.3 mmol m⁻³ to give proper CH₄ fluxes, which are then ~3.3 times smaller than the CH₄ fluxes used in the mass balances of refs. ^5,6.

Because there are only four chamber measurements available for 2016 and one of them was exceptionally low (see ref. ⁶ and Supplementary Note 2.2), the Hallwil relationship is considered here to provide the most reliable estimate of the average k₆₀₀ in Lake Hallwil and therefore applied to calculate the average surface CH₄ flux for April to August 2016, i.e., F_surf = 0.24 mmol m⁻² d⁻¹ (see Table 1 and Supplementary Note 2.2). The reliability of the Hallwil relationship was confirmed by Günthel et al.⁵ and by Hartmann et al.¹¹ comparing different estimates of surface fluxes in the South Basin of Lake Stechlin.

With F_sed = 2.8 mmol m² day⁻¹ and F_surf = 0.24 mmol m² day⁻¹, NOM = 416 mol day⁻¹ and the contribution of NOM to total emissions is NOMC = 17% (Supplementary Table 1 in Supplementary Note 2.3 includes also additional estimates of NOMC). The low value of NOMC suggests that most of CH₄ in the SML originates from the sediments, which is consistent with the δ¹³C isotopic composition of CH₄ in Lake Hallwil⁹. The uppermost CH₄ in the sediment core from the SML is characterized by δ¹³C about –59‰, which corresponds very closely to the δ¹³C of the CH₄ in the open water of the SML ranging from −62‰ to −58‰ (Figs. 4 and 5 both in ref. ⁶). Thus the δ¹³C values suggest that the CH₄ from the uppermost pore water in the sediment of the SML is the source of the CH₄ in the open water and do not indicate a reduction of the δ¹³C expected in case of substantial CH₄ production.

Source: Resources - nature.com