Oxygen transfer across the air-water interface by natural convection in lakes

Oxygen transfer associated with natural convection in lakes and reservoirs was examined in a series of laboratory experiments. A thin, cool surface water layer (2-3 mm, in thickness) was formed by chilling the air overlying a tank of surface area 0.6 m(2) and depth 0.6 m. The surface water layer became gravitationally unstable, resulting in the formation of negatively buoyant thermal plumes, which penetrated through the total depth of the water column. The spatial distribution of oxygen concentration at the air-water interface in the tank was visualized using a fluorescence imaging technique to quantify the oxygen transfer driven by only natural convection. Pyrenebutyric acid (PBA) at a concentration of 3.0 x 10(-6) mole L-1 was used as the fluorophore, and the quenching of the fluorescence by oxygen was used to produce a spatial distribution of dissolved oxygen. A light plane was generated across the tank by the refraction of a laser light beam, and two-dimensional images were continuously acquired with an intensified charge coupled-device (ICCD) camera. Analysis of these images revealed the sinking of cooled water to transport oxygen, and the experiments enabled the quantification of the oxygen transferred from the air into water at a range of heat fluxes. The results confirm that vertical penetration of cold-dense water can be a significant source of oxygen for lakes and reservoirs.