Carbon dioxide and energy fluxes over a small boreal lake in Southern Finland

Abstract

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 (H) flux peaked in the early morning, and upward sensible heat flux at night results in unstable stratification over the lake. Minimum H was measured in the late afternoon, often resulting in adiabatic conditions or slightly stable stratification over the lake. The latent heat flux (LE) showed a different pattern, peaking in the afternoon and having a minimum at night. High correlation (r 2 = 0.75) between H and water-air temperature difference multiplied by wind speed (U) was found, while LE strongly correlated with the water vapor pressure deficit multiplied by U (r 2 = 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 (F CO2 ) averaged over the two open-water periods (0.7 μmol m À2 s À1) 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 À1) shear-induced water turbulence controls the water-air gas transfer efficiency. However, under calm nighttime conditions, F CO2 was poorly correlated with the difference between the water and the equilibrium CO 2 concentrations multiplied by U. 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 F CO2 .

LAKE 2.0: a model for temperature, methane, carbon dioxide and oxygen dynamics in lakes

“…Standard k − turbulence closure is supplemented by parameterizations of internal seiches (Goudsmit, 2002), gravity waves (Mellor, 1989) and a new surface seiche formulation, developing the original concept by Svensson (1978). The model is validated vs. extensive measurement data collected by University of Helsinki at Kuivajärvi Lake (southern Finland) (Miettinen et al, 2015;Mammarella et al, 2015) during the ice-free season of 2013 and including all meteorological variables above lake surface necessary to drive the model. In-water temperature, O 2 , CO 2 and CH 4 vertical profiles from the water column served to validate the model output.…”

“…The measurement setup consisting of an ultrasonic anemometer (USA-1, Metek GmbH, Germany) and an enclosed-path infrared gas analyser (LI-7200, LI-COR Inc., Nebraska, USA) is mounted on a fixed platform situated in the middle of the lake. More details on the measurement platform, the EC system set-up and flux calculation procedures can be found in Mammarella et al (2015). On the platform, a four-way net radiometer (CNR-1) provided the full radiation budget (shortwave and longwave) and a thermistor string of 16 Pt100 resistance thermometers (accuracy of 0.2 • C, depths 0.2, 0.5, 1.0, 1.5, 2.0, 2.…”

“…it is different over different lake depth zones. Therefore, to compare the total CH 4 flux (diffusive plus ebullition) to eddy covariance measurements, it is the bubble flux from the sediment column located approximately below the EC footprint that should be used (for EC footprint at Kuivajärvi Lake, see Mammarella et al, 2015). In our case, it is the deepest sediment column, where the time-average CH 4 ebullition flux reaching the surface constitutes 0.006 µmol (m 2 s) −1 (8 mg (m 2 day) −1 ).…”

“…Kuivajärvi Lake is a significant source of CO 2 (Heiskanen et al, 2014;Miettinen et al, 2015;Mammarella et al, 2015), and significant underestimation of surface concentration by the model (0.39 mg L −1 vs. 2.80 mg L −1 measured) is a serious drawback of the model set-up. As CO 2 in the mixed layer is affected by a large number of processes (BOD, SOD, respiration, photosynthesis, diffusion to the atmosphere), it seems for us difficult to disentangle this problem on a solid physical/biogeochemical basis in this study, and it should be a part of separate research.…”

High-frequency productivity estimates for a lake from free-water CO<sub>2</sub> concentration measurements

“…The lake is dimictic and it is frozen for five months every year on average; the spring turnover occurs immediately after the ice out in late April or early 25 May, and after the turnover a thermocline starts developing. The thermocline deepens until the autumn turnover, and finally the lake freezes over in late November or early December (Heiskanen et al, 2015;Mammarella et al, 2015). A map with the location and bathymetry of the lake is available in the supplemental information (Figg.…”