Drivers of diffusive CH<sub>4</sub> emissions from shallow subarctic lakes on daily to multi-year timescales

Abstract

Lakes and reservoirs contribute to regional carbon budgets via significant emissions of climate forcing trace gases. Here, for improved modelling, we use 8 years of floating chamber measurements from three small, shallow subarctic lakes (2010-2017, n = 1306) to separate the contribution of physical and biogeochemical processes to the turbulencedriven, diffusion-limited flux of methane (CH 4 ) on daily to multi-year timescales. Correlative data include surface water concentration measurements (2009-2017, n = 606), total water column storage (2010-2017, n = 237), and in situ meteorological observations. We used the last to compute nearsurface turbulence based on similarity scaling and then applied the surface renewal model to compute gas transfer velocities. Chamber fluxes averaged 6.9±0.3 mg CH 4 m −2 d −1 and gas transfer velocities (k 600 ) averaged 4.0 ± 0.1 cm h −1 . Chamber-derived gas transfer velocities tracked the powerlaw wind speed relation of the model. Coefficients for the model and dissipation rates depended on shear production of turbulence, atmospheric stability, and exposure to wind. Fluxes increased with wind speed until daily average values exceeded 6.5 m s −1 , at which point emissions were suppressed due to rapid water column degassing reducing the water-air concentration gradient. Arrhenius-type temperature functions of the CH 4 flux (E a = 0.90 ± 0.14 eV) were robust (R 2 ≥ 0.93, p<0.01) and also applied to the surface CH 4 concentration (E a = 0.88 ± 0.09 eV). These results imply that emissions were strongly coupled to production and supply to the water column. Spectral analysis indicated that on timescales shorter than a month, emissions were driven by wind shear whereas on longer timescales variations in water temperature governed the flux. Long-term monitoring efforts are essential to identify distinct functional relations that govern flux variability on timescales of weather and climate change.

Volatile organic compound fluxes in a subarctic peatland and lake

“…Most of the other published studies derived ecosystemscale isoprene fluxes with measurement chambers attached to the ground, enclosing the vegetation and the soil surface together. Pioneering VOC work started in boreal Sphagnum fens of Sweden and Finland with static chambers, finding isoprene fluxes of up to 9 nmol m −2 s −1 , which is in range with our measurements (Janson et al, 1999;Janson and De Serves, 1998). At a subarctic fen dominated by Eriophorum spp.…”

“…The fluxes of CO 2 and CH 4 (as well as H 2 O) were not the focus of this study, and, moreover, their temporal patterns and environmental drivers over several years at the same site have been examined in detail elsewhere (Jammet et al, 2015(Jammet et al, , 2017Jansen et al, 2019aJansen et al, , 2020. Here, we mainly included them to contextualize the VOC fluxes and thus provide a broader overview of the trace gas exchange of our fen and Villasjön during our two measurement periods.…”

Temperature Proxies as a Solution to Biased Sampling of Lake Methane Emissions

“…Proxies were more efficient at quantifying diffusive fluxes, which displayed no hysteresis (Figures 3b and 3c). Gas transfer models (Figure 3b) required more sampling days than floating chambers (Figure 3c), likely due to diel variability of Δ[CH 4 ] (Jansen et al, 2020), which was integrated by the 24‐hr chamber deployments but not resolved with discrete water sampling. The use of in situ CH 4 sensors would mitigate that problem (Encinas Fernández et al, 2014; Erkkilä et al, 2018).…”

“…Lakes Villasjön, Inre Harrsjön, and Mellersta Harrsjön are of postglacial origin, are 0.17, 0.02, and 0.01 km 2 in area and have mean depths of 0.7, 2.0, and 1.9 m, respectively (Wik et al, 2013), as is typical of most lakes north of 50°N (Cael et al, 2017; Wik, Varner, et al, 2016). The ice‐free period lasts from May through October (∼161 days, 2009–2018), during which the lakes frequently mix to the bottom (Jansen et al, 2020). Ebullition accounts for 83%, 48%, and 66% of the ice‐free CH 4 flux in Villasjön, Inre, and Mellersta Harrsjön, respectively, and the remainder by diffusion (2009–2017, Jansen et al, 2019).…”

Volatile Organic Compound fluxes in a subarctic peatland and lake

“…The fluxes of CO 2 and CH 4 (as well as H 2 O) were not the focus of this study and, moreover, their temporal patterns and environmental drivers over several years at the same site have been examined in detail elsewhere (Jammet et al, 2015(Jammet et al, , 2017Jansen et al, 2019Jansen et al, , 2020. Here, we mainly included them to contextualize the VOC fluxes and thus provide a broader overview of the trace gas exchange of our fen and Villasjön during our two measurement periods.…”