Drivers of diffusive CH<sub>4</sub> emissions from shallow subarctic lakes on daily to multi-year timescales
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Published:2020-04-08
Issue:7
Volume:17
Page:1911-1932
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ISSN:1726-4189
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Container-title:Biogeosciences
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language:en
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Short-container-title:Biogeosciences
Author:
Jansen JoachimORCID, Thornton Brett F.ORCID, Cortés Alicia, Snöälv Jo, Wik Martin, MacIntyre Sally, Crill Patrick M.ORCID
Abstract
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 turbulence-driven,
diffusion-limited flux of methane (CH4) 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 near-surface 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 CH4 m−2 d−1 and gas transfer
velocities (k600) averaged 4.0±0.1 cm h−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 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 CH4 flux (Ea′=0.90±0.14 eV) were robust (R2≥0.93, p<0.01) and also applied to
the surface CH4 concentration (Ea′=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.
Funder
Division of Arctic Sciences Vetenskapsrådet Natural Sciences and Engineering Research Council of Canada
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
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