Contribution of oxic methane production to surface methane emission in lakes and its global importance

Abstract

Recent discovery of oxic methane production in sea and lake waters, as well as wetlands, demands re-thinking of the global methane cycle and re-assessment of the contribution of oxic waters to atmospheric methane emission. Here we analysed system-wide sources and sinks of surface-water methane in a temperate lake. Using a mass balance analysis, we show that internal methane production in well-oxygenated surface water is an important source for surface-water methane during the stratified period. Combining our results and literature reports, oxic methane contribution to emission follows a predictive function of littoral sediment area and surface mixed layer volume. The contribution of oxic methane source(s) is predicted to increase with lake size, accounting for the majority (>50%) of surface methane emission for lakes with surface areas >1 km 2 .

Sub-oxycline methane oxidation can fully uptake CH4 produced in sediments: case study of a lake in Siberia

“…While the hypolimnion is dominated by methanotrophy and hydrogenotrophic methanogenesis, the epilimnion is dominated by CH 4 originating from acetoclastic production, in situ or transferred from surrounding terrestrial ecosystems. The segregation between the CH 4 cycle in the epilimnion and the hypolimnion has been suggested by several reports 5,8,10,12,13 and used as a supporting evidence that epilimnetic CH 4 is locally produced 8,[10][11][12][13]27 or transported from the littoral zone 1,8,28 . In addition to similar conclusions, the present work was based on an innovative high resolution and sensitive method for determining C CH4 , which allowed the NMPR profile to be created and demonstrated that methanotrophy can occur well below the oxycline and euphotic zone of a lake.…”

“…Thus, ebullitive transfer of CH 4 from the hypolimnion to the epilimnion, is unlikely although not discountable. The CH 4 found at the epilimnion might be considered as the product of local oxic production and/or lateral transport from the littoral zone 1,8,[10][11][12][13] .…”

“…Although this has been the prevailing theory, several other CH 4 cycle processes have been discovered. For instance, CH 4 production in the epilimnion under aerobic conditions has been observed in several aquatic ecosystems 8,[10][11][12][13] . Another process previously reported is CH 4 oxidation below the oxycline, which is typically associated with microaerophilic conditions 4 , oxygenic photosynthesis 4,14 , or, in some cases, assumed to be anaerobic 15 .…”

A Fast-response automated gas equilibrator (FaRAGE) for continuous in situ measurement of methane dissolved in water

“…In many deep stratified lakes, a sharp vertical gradient below the thermocline can develop in the anoxic hypolimnion (mM range) (Encinas Fernández et al, 2014;Liu et al, 1996). In contrast, in some stratified lakes with a fully oxygenated hypolimnion CH 4 can accumulate above the thermocline (~M range) (Grossart et al, 2011;Donis et al, 2017;Günthel et al, 2019). The concentration of dissolved CH 4 is also regulated by loss due to oxidation and emission to the atmosphere (Bastviken et al, 2004;Juutinen et al, 2009).…”

“…The rise and fall of lake CH 4 concentration often show strong seasonality that are driven primarily by thermal stratification (Encinas Fernández et al, 2014) and phytoplankton dynamics (Günthel et al, 2019). While the build-up of hypolimnetic CH storage is a slow process that is closely related to the development of lake hypoxia, the epilimnetic CH 4 maximum can be highly variable even at a daily basis as it is strongly affected by phytoplankton dynamics (Günthel et al, 2019;Hartmann et al, 2020;Bižić et al, 2020). In addition, storms can act as another driver for short-term CH 4 dynamics in the lake because it often leads to higher evasion rates caused by strong vertical turbulent mixing (Zimmermann et al, 2019) and enhanced horizontal transport (Fernández et al, 2016).…”