Seasonally-stratified temperate lakes are a source of carbon dioxide to the atmosphere during autumn overturning as CO2 trapped below the thermocline becomes available to the surface for release to the atmosphere. Wind-only based parameterizations of the gas transfer coefficient do not explicitly capture buoyancy-induced mixing in the water column caused by heat loss to the atmosphere, and CO2 fluxes may therefore be underestimated during fall cooling. We made continuous eddy covariance flux measurements of momentum, sensible and latent heat, and CO2 over a ~600 ha temperate lake (Lake Pleasant, maximum depth ~24 m) in southwestern Adirondack Park, New York from mid-September to mid-October 2010 from a moored pontoon boat. Continuous measurements of the vertical profile of pCO2 in the water column were made in situ, and the water column thermal structure was measured using thermistor chains. The spatial variability (horizontal and vertical) of pCO2 in the lake was characterized using a roving profiling system. At the beginning of the study interval, pCO2 varied from 500 ppm at the surface to > 3000 ppm below the thermocline. By the end of the campaign the vertical profile of pCO2 had changed markedly, with nearly uniform, high pCO2 throughout the water column (Figure 1). The elevated surface water pCO2 increased CO2 emission to the atmosphere. The measured gas exchange coefficient was considerably higher than wind-only parameterizations, particularly when buoyancy was an important source of turbulence at the air-water interface.
Figure 1: Lake-atmosphere coupling measured from a pontoon boat moored on Lake Pleasant, NY from 16 September to 11 October, 2010: a) wind speed adjusted to 10 m height; b) surface water temperature at 10 cm depth (blue) and air temperature at 2 m height (green); and c) vertical profile of pCO2 (ppm). Cooling periods when the lake heat loss to the atmosphere was >50 W m-2 are indicated by the horizontal blue bar along the upper border of panel a.