Influence of water column stratification and mixing patterns on the fate of methane produced in deep sediments of a small eutrophic lake

Abstract

Methane (CH4), a potent greenhouse gas, is produced in and emitted from lakes at globally significant rates. The drivers controlling the proportion of produced CH4 that will reach the atmosphere, however, are still not well understood. We sampled a small eutrophic lake (Soppensee, Switzerland) in 2016–2017 for CH4 concentrations profiles and emissions, combined with water column hydrodynamics to investigate the fate of CH4 produced in hypolimnetic sediments. Using a mass balance approach for the periods between April and October of both years, net CH4 production rates in hypolimnetic sediments ranged between 11.4 and 17.7 mmol m−2 d−1, of which 66–88% was stored in the hypolimnion, 13–27% was diffused to the epilimnion, and 6–7% left the sediments via ebullition. Combining these results with a process‐based model we show that water column turbulent diffusivity (K z) had a major influence on the fate of produced CH4 in the sediments, where higher K z values potentially lead to greater proportion being oxidized and lower K z lead to a greater proportion being stored. During fall when the water column mixes, we found that a greater proportion of stored CH4 is emitted if the lake mixes rapidly, whereas a greater proportion will be oxidized if the water column mixes more gradually. This work highlights the central role of lake hydrodynamics in regulating CH4 dynamics and further suggests the potential for CH4 production and emissions to be sensitive to climate‐driven alterations in lake mixing regimes and stratification.

Whole‐ecosystem oxygenation experiments reveal substantially greater hypolimnetic methane concentrations in reservoirs during anoxia

“…Finally, we also note that the hypolimnion, as defined in this study (> 6.5 m in FCR; > 7 m in BVR), represents 5% and 11% of the total reservoir volume for FCR and BVR, respectively. Thus, while the hypolimnion likely plays a disproportionate role in driving the GHG budgets of these two ecosystems (Bartosiewicz et al 2019;Vachon et al 2019), this remains unquantified.…”

Drivers of diffusive lake CH<sub>4</sub> emissions on daily to multi-year time scales

“…In lakes, the air-water concentration difference driving the flux (Eq. 1) is further impacted by abiotic factors that dissociate production from emission rates, such as hydrologic inputs of terrestrially produced CH4 (Miettinen et al, 2015;Murase et al, 2003;Paytan et al, 2015), redistribution of dissolved gas in the water column (DelSontro et al, 2017;Hofmann, 2013) and storage-and-release cycles associated with transient stratification (Czikowsky et al, 2018;Jammet et al, 2017;Vachon et al, 2019). From these interacting functional dependencies emerge complex responses of the flux to biotic and abiotic factors.…”

“…Concentration gradients may also have been caused by physical processes, such as upwelling due to thermocline tilting (Heiskanen et al, 2014). Higher resolution measurements, for example with automated equilibration systems (Erkkilä et al, 2018;Natchimuthu et al, 2016), are needed to assess how much of the spatial and diel patterns of the CH4 concentration can be explained by physical drivers such as gas transfer and mixed layer deepening (Eugster et al, 2003;Vachon et al, 2019), or by biological processes such as methanogenesis and microbial oxidation (Ford et al, 2002). The distinct spectral peaks of U10 and temperature (Fig.…”

Methane emission offsets carbon dioxide uptake in a small productive lake

“…The lake was strongly thermally stratified from May to October each year, after which the water column mixed partially in winter 2016/2017 (with ice cover) and completely in winter 2017/2018. Vertical profiles of water temperature and conductivity, as well as a more detailed physical description of the water column, are provided in Vachon et al (2019 a ).…”