Diel cycle of lake‐air CO₂ flux from a shallow lake and the impact of waterside convection on the transfer velocity

Abstract

Two years of eddy covariance measurements of lake carbon dioxide (CO 2 ) fluxes reveal a diel cycle with higher fluxes during night. Measurements of partial pressure in the air (pCO 2a ) and in the water (pCO 2w ), during 4 months, show that the high nighttime fluxes are not explained by changes in the difference between pCO 2a and pCO 2w . Analyzing the transfer velocity (k 600,meas ), which is a measure of the efficiency of the gas transfer, with respect to wind speed, shows that variations in wind speed do not explain the diel cycle. During nighttime, when the fluxes are high, the wind is normally low. Thus, a solely wind-based parameterization of the transfer velocity (k u,CC ) results in large errors compared to k 600,meas , especially for wind speeds lower than 6 m s À1 . The mean absolute percentage error between k u,CC and k 600,meas is 79%.By subtracting k u,CC from k 600,meas , we investigate how waterside convection influence k 600,meas . Our results show that the difference (k 600,meas À k u,CC ) increases with increasing waterside convection. Separating the transfer velocity parameterization in two parts, one depending on the wind speed and one depending on waterside convection, the mean absolute percentage error compared to the measurements reduces to 22%. The results in this paper show that the high nighttime CO 2 fluxes can, to a large extent, be explained by waterside convection and that a transfer velocity parameterization based on both wind speed and waterside convection better fits the measurements compared to a parameterization based solely on wind speed.

Methane and carbon dioxide fluxes over a lake: comparison between eddy covariance, floating chambers and boundary layer method

“…The same result of k CC giving lower fluxes than EC was found also in other studies (e.g. Heiskanen et al, 2014;Mammarella et al, 2015;Podgrajsek et al, 2015) and the use of this model in global carbon budget estimates may therefore be questionable (e.g. Raymond et al, 2013).…”

“…A1a as the convective term C 2 w * increases towards night-time causing higher gas transfer coefficient k HE and thus higher flux as well. Podgrajsek et al (2015) argued that the main driver for enhanced night-time gas exchange is convection, and they did not find a correlation with the concentration difference [CO 2 ]. However, we find that [CO 2 ] also increases during night-time due to the absence of algal photosynthesis.…”

Year-round CH<sub>4</sub> and CO<sub>2</sub> flux dynamics in two contrasting freshwater ecosystems of the subarctic

“…Wind-driven turbulence is a recognized driver of diffusion-limited exchange of CO 2 and CH 4 across the lake-water interface (Sebacher et al, 1983;Wanninkhof et al, 1985). Convective mixing due to the cooling of the lake surface or following the breakdown of thermal stratification in the water column can increase advection of gas-rich water from the lake bottom, thus enhancing the diffusion-limited release of gases to the atmosphere (Eugster, 2003;MacIntyre et al, 2010;Podgrajsek et al, 2015), especially if the main source of those gases is the sediments, as is the case for CH 4 .…”

Consequences of gas flux model choice on the interpretation of metabolic balance across 15 lakes

“…For instance, in low-wind environments, such as the bog lakes, gas exchange can be driven largely by diel heating and cooling (Read et al 2012, Heiskanen et al 2014, Podgrajsek et al 2015. R12S, T14, and H14, which incorporate a convective forcing component, are potentially able to reproduce the diel variability in k caused by buoyant mixing (Fig.…”