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

Jansen, Joachim; Thornton, Brett F.; Cortés, Alicia; Snöälv, Jo; Wik, Martin; MacIntyre, Sally; Crill, Patrick M.

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, inline-formulan=1306) to separate the contribution of physical and biogeochemical processes to the turbulence-driven, diffusion-limited flux of methane (inline-formulaCH4) on daily to multi-year timescales. Correlative data include surface water concentration measurements (2009–2017, inline-formulan=606), total water column storage (2010–2017, inline-formulan=237), and in situ meteorological observations. We used the last to compute near-surface turbulence based on similarity scaling and then applied the surface renewal model to compute gas transfer velocities. Chamber fluxes averaged inline-formula6.9±0.3 mg inline-formulaCH4 minline-formula−2 dinline-formula−1 and gas transfer velocities (inline-formulak600) averaged inline-formula4.0±0.1 cm hinline-formula−1. Chamber-derived gas transfer velocities tracked the power-law 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 sinline-formula−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 inline-formulaCH4 flux (inline-formula M16inlinescrollmathml E normal a = normal 0.90 ± normal 0.14 81pt14ptsvg-formulamathimg72587c844285b35daab55b4019f9a7c5 bg-17-1911-2020-ie00001.svg81pt14ptbg-17-1911-2020-ie00001.png  eV) were robust (inline-formulaR2≥0.93, inline-formulap<0.01) and also applied to the surface inline-formulaCH4 concentration (inline-formula M20inlinescrollmathml E normal a = normal 0.88 ± normal 0.09 81pt14ptsvg-formulamathimgdd381a15b8b2961c6b726097e41b117a bg-17-1911-2020-ie00002.svg81pt14ptbg-17-1911-2020-ie00002.png  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.

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Jansen, Joachim / Thornton, Brett F. / Cortés, Alicia / et al: Drivers of diffusive CH4 emissions from shallow subarctic lakes on daily to multi-year timescales. 2020. Copernicus Publications.

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