Evaluation of the methane paradox in four adjacent pre-alpine lakes across a trophic gradient

Abstract

Contrasting the paradigm that methane is only produced in anoxic conditions, recent discoveries show that oxic methane production (OMP, aka the methane paradox) occurs in oxygenated surface waters worldwide. OMP drivers and their contribution to global methane emissions, however, are not well constrained. In four adjacent pre-alpine lakes, we determine the net methane production rates in oxic surface waters using two mass balance approaches, accounting for methane sources and sinks. We find that OMP occurs in three out of four studied lakes, often as the dominant source of diffusive methane emissions. Correlations of net methane production versus chlorophyll-a, Secchi and surface mixed layer depths suggest a link with photosynthesis and provides an empirical upscaling approach. As OMP is a methane source in direct contact with the atmosphere, a better understanding of its extent and drivers is necessary to constrain the atmospheric methane contribution by inland waters.

Fig. 1. Conceptual schematic of the CH₄ budget components in the surface mixed layer (SML) and methodological approaches.

CH₄ mass balance components: diffusive CH₄ emissions to the atmosphere (F_a), vertical transport (F_z), bubble dissolution (R_dis), littoral sediment flux (F_s). The net CH₄ production rate (P_net) in the SML is estimated using a 1-D lateral transport model and a 0-D full-scale mass balance in a and b, respectively. P_net is the balance between oxic CH₄ production (OMP, adds CH₄) and CH₄ oxidation (MOx, removes CH₄). The full-scale mass balance assumes the SML as a well-mixed reactor where each component is based on measured values. The lateral transport model also used in situ measurements but estimates the diffusive flux to the atmosphere using the mass transfer coefficient (k_CH4) and P_net rates are obtained by finding the simulated transect CH₄ concentrations (C(r)) that best-fit the measured CH₄ concentrations.

Fig. 2. Surface CH₄ concentrations along the transects sampled in each lake.

a Lac de Bretaye, b Lac Noir, c Lac des Chavonnes, and d Lac Lioson. Lines represent the CH₄ concentration simulated using the lateral transport model and dots are the measured values. Since the lateral transport model assumes that the CH₄ concentrations in the SML are radially symmetric, the concentrations are shown from shore to center. The bathymetry profile along the transects is shown in Supplementary Fig. 4.

Fig. 3. P_net rates estimations in the surface mixed layer of each lake using two approaches.

The full-scale mass balance (P_net,fs; filled boxes) and lateral transport model (P_net,lt; open boxes). The lakes were divided as a eutrophic and b oligotrophic lakes. Boxes show the first and third quartiles with the median (line), whiskers extend to most extreme data point within 1.5 times the interquartile range from the box. The white dot represents the average of the P_net distribution. Note different scales on y-axes of the two panels.

Fig. 4. Contribution to diffusive atmospheric CH₄ emissions from each component of the CH₄ budget.

The sediment flux (F_s), diffusive flux from hypolimnion (F_z), bubble dissolution (R_dis), and net production rates (P_net) in the SML of Lac de Bretaye (BRE), Lac Noir (NOI), Lac des Chavonnes (CHA) and Lac Lioson (LIO). The lakes were divided as a eutrophic and b oligotrophic lakes. The results from the full-scale mass balance were used as representative P_net rates of the studied lakes.

Fig. 5. Linking net CH₄ production (P_net) in the surface mixed layer (SML) with trophic variables.

a Relationship between P_net and light climate (LC, m m⁻¹) and trophic state. Per lake, the minimum P_net rate ($P_{\mathrm{net},\min }$) and the minimum LC (LC_min) were subtracted to be able to compare the slope of each curve. P_net becomes more independent of LC in more oligotrophic lakes. b Interaction between P_net (mmol m⁻³ d⁻¹) and the average surface concentration of chlorophyll-a (Chla, mg m⁻³), LC (m m⁻¹) and Secchi depth (Z_s, m) suggest a direct role of photosynthesis on OMP. Specific production/oxidation rate calculated as P_net normalized by the average surface concentration of CH₄ ($C_{{\mathrm{CH}}_4}$ mmol m⁻³) versus Chla × light climate ($LC=2.5\frac{Z_{\mathrm{s}}}{H_{\mathrm{SML}}}$) × Z_s; where H_SML is the surface mixed layer depth. Chla was obtained from fluoroprobe profiles measured at the center of the lake. All the parameters were calculated at each sampling campaign. The results from the full-scale mass balance were used as representative P_net rates of the studied lakes.