Publication Date:
2017-03-02
Description:
Freshwaters bring a notable contribution to the global carbon budget by emitting both carbon dioxide (CO2) and methane (CH4) to the atmosphere. Global estimates of freshwater emissions traditionally use a wind speed based gas transfer coefficient, kCC, for calculating diffusive flux with the boundary layer method (BLM). We compared CH4 and CO2 fluxes from BLM with kCC and two other gas transfer coefficients (kTE and kHE), that include the effects of water-side cooling to the gas transfer besides shear-induced turbulence, with simultaneous eddy covariance (EC) and floating chamber (FC) fluxes during a 16-day measurement campaign in September 2014 at Lake Kuivajärvi in Finland. The measurements included both lake stratification and water column mixing periods. A clear increase in both CH4 and CO2 fluxes was detected right after turnover started. Results show that kCC model underestimates both CH4 and CO2 fluxes compared to EC measurements and the more recent model kTE gives the best fit with EC fluxes. In the stratified period, kHE and kCC used in BLM calculation have similar diurnal variation for CO2 flux, while kTE gives an opposite diurnal cycle. This is probably due to water friction velocity derived from EC measurements in kTE model instead of direct waterside turbulence measurements in this study. In the mixing period all models agree well with each other on diurnal variation for both CH4 and CO2. We highly recommend using up to date gas transfer models, instead of kCC, for better flux estimates. FC measurements did not detect spatial variation in either CH4 or CO2 flux over Lake Kuivajärvi. EC measurements, on the other hand, did not show any spatial variation in CH4 fluxes, but a clear difference between CO2 fluxes from shallower and deeper areas. Fluxes measured with FC were systematically higher than EC fluxes, partly due to deficiencies in the EC method.
Print ISSN:
1810-6277
Electronic ISSN:
1810-6285
Topics:
Biology
,
Geosciences
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