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.

Small ponds, numerous throughout the Arctic, are often supersaturated with climate-forcing trace gases. Improving estimates of emissions requires quantifying (1) their mixing dynamics and (2) near-surface turbulence which would enable emissions. To this end, we instrumented an arctic pond (510 m 2 , 1 m deep) with a meteorological station, a thermistor array, and a vertically oriented acoustic Doppler velocimeter. We contrasted measured turbulence, as the rate of dissipation of turbulent kinetic energy, e, with values predicted from Monin-Obukhov similarity theory (MOST) based on wind shear as u *w , the water friction velocity, and buoyancy flux, b, under cooling. Stratification varied over diel cycles; the thermocline upwelled as winds changed allowing ventilation of near-bottom water. Near-surface temperature stratification was up to 78C per meter. With respect to predictions from MOST: (1) With positive b under heating and strong near-surface stratification, turbulence was suppressed; (2) under heating with moderate stratification and under cooling with light to moderate winds, measured e was in agreement with MOST; (3) under cooling with no wind and when surface currents had ceased, as occurred 20% of the time, turbulence was measurable and predicted from b. Near-surface turbulence was enhanced under cooling and light winds relative to that under a neutral atmosphere due to higher values of drag coefficients under unstable atmospheres. Small ponds are dynamic systems with wind-induced thermocline tilting enabling vertical exchanges. Near-surface turbulence, similar to that in larger systems, can be computed from surface meteorology enabling accurate estimates of gas transfer coefficients and emissions. Matveev et al. 2016). Concentrations of CO 2 and CH 4 are supersaturated in surface waters and can be orders of magnitude higher near the bottom. They have well developed microbial communities including methanotrophs and methanogens (Crevecoeur et al. 2015(Crevecoeur et al. , 2016 and process DOC from terrestrial and algal sources (Roiha et al. 2016). Carbon budgets from northern water bodies, however, are based on infrequent sampling which does not take into account the diel and synoptic variability in mixing which can moderate emissions (Liu et al. 2016;Wik et al. 2016b). Studies of the mixing dynamics of ponds are rare, yet the contribution of ponds to carbon cycles depends, in part, on the frequency of events which bring dissolved gases to the air-water interface and on the magnitude of near surface turbulence which enables fluxes across the air-water interface. Time series temperature and dissolved oxygen measurements in arctic ponds indicate considerable temporal variability in near surface temperatures in summer and partial mixing of the water column in spring and fall (Laurion et al. 2010;Deshpande et al. 2015). It is not known whether near surface stratification damps turbulence or how deep and intense the mixing is at night. Wedderburn numbers have been computed for three subarctic ponds an...

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