Air-lake methane flux (FCH 4 ) and partial pressure of methane in the atmosphere (pCH 4a ) were measured using the eddy covariance method over a Swedish lake for an extended period. The measurements show a diurnal cycle in both FCH 4 and pCH 4a with high values during nighttime (FCH 4 ≈ 300 nmol m À2 s À1 , pCH 4a ≈ 2.5 μatm) and low values during day (FCH 4 ≈ 0 nmol m À2 s À1 , pCH 4a ≈ 2.0 μatm) for a large part of the data set. This diurnal cycle persist in all open water season; however, the magnitude of the diurnal cycle is largest in the spring months. Estimations of buoyancy in the water show that high nighttime fluxes coincide with convective periods. Our interpretation of these results is that the convective mixing enhances the diffusive flux, in analogy to previous studies. We also suggest that the convection may bring methane-rich water from the bottom to the surface and trigger bubble release from the sediment. A diurnal cycle is not observed for all convective occasions, indicating that the presence of convection is not sufficient for enhanced nighttime flux; other factors are also necessary. The observed diurnal cycle of pCH 4a is explained with the variation of FCH 4 and a changing internal boundary layer above the lake. The presence of a diurnal cycle of FCH 4 stresses the importance of making long-term continuous flux measurements. A lack of FCH 4 measurements during night may significantly bias estimations of total CH 4 emissions from lakes to the atmosphere.

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.

Abstract. Fluxes of carbon dioxide (CO 2 ) and methane (CH 4 ) from lakes may have a large impact on the magnitude of the terrestrial carbon sink. Traditionally lake fluxes have been measured using the floating chamber (FC) technique; however, several recent studies use the eddy covariance (EC) method. We present simultaneous flux measurements using both methods at lake Tämnaren in Sweden during field campaigns in 2011 and 2012. Only very few similar studies exist. For CO 2 flux, the two methods agree relatively well during some periods, but deviate substantially at other times. The large discrepancies might be caused by heterogeneity of partial pressure of CO 2 (pCO 2w ) in the EC flux footprint. The methods agree better for CH 4 fluxes. It is, however, clear that short-term discontinuous FC measurements are likely to miss important high flux events.

The hydrodynamics within small boreal lakes have rarely been studied, yet knowing whether turbulence at the air-water interface and in the water column scales with metrics developed elsewhere is essential for computing metabolism and fluxes of climate-forcing trace gases. We instrumented a humic, 4.7 ha, boreal lake with two meteorological stations, three thermistor arrays, an infrared (IR) camera to quantify surface divergence, obtained turbulence as dissipation rate of turbulent kinetic energy (ε) using an acoustic Doppler velocimeter and a temperature-gradient microstructure profiler, and conducted chamber measurements for short periods to obtain fluxes and gas transfer velocities (k). Near-surface ε varied from 10 −8 to 10 −6 m 2 s −3 for the 0-4 m s −1 winds and followed predictions from Monin-Obukhov similarity theory. The coefficient of eddy diffusivity in the mixed layer was up to 10 −3 m 2 s −1 on the windiest afternoons, an order of magnitude less other afternoons, and near molecular at deeper depths. The upper thermocline upwelled when Lake numbers (L N) dropped below four facilitating vertical and horizontal exchange. k computed from a surface renewal model using ε agreed with values from chambers and surface divergence and increased linearly with wind speed. Diurnal thermoclines formed on sunny days when winds were < 3 m s −1 , a condition that can lead to elevated near-surface ε and k. Results extend scaling approaches developed in the laboratory and for larger water bodies, illustrate turbulence and k are greater than expected in small wind-sheltered lakes, and provide new equations to quantify fluxes.

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Fluxes of methane, CH 4 , were measured with the eddy covariance (EC) method at a small boreal lake in Sweden. ). Monthly mean values of CH 4 flux measured with the EC method were compared with fluxes measured with floating chambers (FC) and were in average 62% higher over the whole study period. The difference was greatest in April partly because EC, but not FC, accounted for fluxes due to ice melt and a subsequent lake mixing event. A footprint analysis revealed that the EC footprint included primarily the shallow side of the lake with a major inlet. This inlet harbors emergent macrophytes that can mediate high CH 4 fluxes. The difference between measured EC and FC fluxes can hence be explained by different footprint areas, where the EC system "sees" the part of the lake presumably releasing higher amounts of CH 4 . EC also provides more frequent measurements than FC and hence more likely captures ebullition events. This study shows that small lakes have CH 4 fluxes that are highly variable in time and space. Based on our findings we suggest to measure CH 4 fluxes from lakes as continuously as possible and to aim for covering as much of the lakes surface as possible, independently of the selected measuring technique.Methane, CH 4 , is an important greenhouse gas with approximately 25 times higher global warming potential than carbon dioxide, CO 2 , by mass and considering the effect of a single pulse emission over a 100 year period (Forster et al. 2007) . Recent studies by Bastviken et al. (2011) andCiais et al. 2013 highlighted the huge CH 4 emissions from lakes, estimated to correspond to 25% of the CO 2 equivalents sequestered by the terrestrial carbon sink reported by IPCC. However, these studies also recognized the large uncertainty of the measurements and the need for development of more representative measurement approaches.CH 4 is mainly produced in the anoxic lake sediments and higher temperatures will normally result in higher CH 4 production (e.g., Duc et al. 2010;Marotta et al. 2014). From the sediment, CH 4 can be transported to the atmosphere along different pathways; diffusion, storage transport, ebullition (bubble flux), and transport through plants (e.g., Bastviken et al. 2004). The diffusive flux over the water-air interface is driven by the concentration difference between the water and the air and controlled by the transfer velocity. The transfer velocity describes the efficiency of the gas transfer and is controlled by, e.g., wind speed, and waterside convection (e.g., Rutgersson and Smedman 2010;Podgrajsek et al. 2014b).The diffusive flux is substantially reduced by consumption of CH 4 in oxic sediments or waters by methane oxidizing bacteria (Bastviken 2009). Storage transport is a special case of diffusive transport: If a lake is stratified with anoxic bottom water, a large amount of CH 4 can be stored in the anoxic water layers (Michmerhuizen et al. 1996;Riera et al. 1999;Bastviken et al. 2004). With mixing of the lake, this CH 4 rich water is transported from the bottom to the...

The short-term (hourly and daily) variation in chromophoric dissolved organic matter (CDOM) in lakes is largely unknown. We assessed the spectral characteristics of light absorption by CDOM in a eutrophic, humic shallow mixed lake of temperate Sweden at a high-frequency (30 min) interval and during a full growing season (May to October). Physical time series, such as solar radiation, temperature, wind, and partial pressures of carbon dioxide in water and air, were measured synchronously. We identified a strong radiation-induced summer CDOM loss (25 to 50%) that developed over 4 months, which was accompanied by strong changes in CDOM absorption spectral shape. The magnitude of the CDOM loss exceeded subhourly to daily variability by an order of magnitude. Applying Fourier analysis, we demonstrate that variation in CDOM remained largely unaffected by rapid shifts in weather, and no apparent response to in-lake dissolved organic carbon production was found. In autumn, CDOM occasionally showed variation at hourly to daily time scales, reaching a maximum daily coefficient of variation of 15%. We suggest that lake-internal effects on CDOM are quenched in humic lake waters by dominating effects associated with imported CDOM and solar exposure. Since humic lake waters belong to one of the most abundant lake types on Earth, our results have important implications for the understanding of global CDOM cycling.

Accounting for temporal changes in carbon dioxide (CO2) effluxes from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of flux measurements of CO2 based on the eddy covariance method from 13 lakes and reservoirs in the Northern Hemisphere, and quantify dynamics at multiple temporal scales. We found pronounced sub-annual variability in CO2 flux at all sites. By accounting for diel variation, only 11% of site-months were net daily sinks of CO2. Annual CO2 emissions had an average of 25% (range 3-58%) interannual variation. Similar to studies on streams, nighttime emissions regularly exceeded daytime emissions. Biophysical regulations of CO2 flux variability were delineated through mutual information analysis. Sample analysis of CO2 fluxes indicate the importance of continuous measurements. Better characterization of short- and long-term variability is necessary to understand and improve detection of temporal changes of CO2 fluxes in response to natural and anthropogenic drivers. Our results indicate that existing global lake carbon budgets relying primarily on daytime measurements yield underestimates of net emissions.