Physical Controls on Carbon Flux from a Temperate Lake During Autumn Cooling

            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.