Carbon dioxide and energy fluxes over a small boreal lake in Southern Finland. Issue 7 (23rd July 2015)

• Record Type: Journal Article
• Authors: Mammarella, Ivan; Nordbo, Annika; Rannik, Üllar; Haapanala, Sami; Levula, Janne; Laakso, Heikki; Ojala, Anne; Peltola, Olli; Heiskanen, Jouni; Pumpanen, Jukka; Vesala, Timo
• Is Part Of: Journal of geophysical research. Volume 120:Issue 7(2015:Sep.)
• Page Start: 1296
• Page End: 1314
• Publication Date: 2015-07-23
• Subjects: Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1002/2014JG002873
• Languages: English
• ISSNs: 2169-8953

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>Dynamics of carbon dioxide and energy exchange over a small boreal lake were investigated. Flux measurements have been carried out by the eddy covariance technique during two open‐water periods (June–October) at Lake Kuivajärvi in Finland. Sensible heat (<italic>H</italic>) flux peaked in the early morning, and upward sensible heat flux at night results in unstable stratification over the lake. Minimum <italic>H</italic> was measured in the late afternoon, often resulting in adiabatic conditions or slightly stable stratification over the lake. The latent heat flux (<italic>LE</italic>) showed a different pattern, peaking in the afternoon and having a minimum at night. High correlation (<italic>r</italic><sup>2</sup> = 0.75) between <italic>H</italic> and water‐air temperature difference multiplied by wind speed (<italic>U</italic>) was found, while <italic>LE</italic> strongly correlated with the water vapor pressure deficit multiplied by <italic>U</italic> (<italic>r</italic><sup>2</sup> = 0.78). Monthly average values of energy balance closure ranged between 70 and 99%. The lake acted as net source of carbon dioxide, and the measured flux (<italic>F</italic><sub>CO2</sub>) averaged over the two open‐water periods (0.7 µmol m<sup>−2</sup> s<sup>−1</sup>) was up to 3 times higher than those reported in other studies. Furthermore, it was found that during period of high wind speed (>3 m s<sup>−1</sup>) shear‐induced water turbulence controls the water‐air gas transfer efficiency. However, under calm nighttime conditions, <italic>F</italic><sub>CO2</sub> was poorly correlated with the difference between the water and the equilibrium CO<sub>2</sub> concentrations multiplied by <italic>U</italic>. Nighttime cooling of surface water enhances the gas transfer efficiency through buoyancy‐driven turbulent mixing, and simple wind speed‐based transfer velocity models strongly underestimate <italic>F</italic><sub>CO2</sub>.</p> </abstract>