Wind shear is indeed the dominant driving mechanism for gas transfer in oceans and lakes with moderate or strong wind speeds. However, there exist many lakes that are topographically sheltered, giving them protection against strong wind (wind speeds below 3 m s~ ) and limiting their wave generation due to their smaller fetch length when compared to the ocean. Such sheltered lakes are normally characterized by a well-defined thermal structure and undergo diurnal temperature fluctuations due to daytime heating and nighttime cooling. During daytime, the main oxygen supply comes from photosynthetic production in the surface layer and often leads to oversaturation of oxygen concentration. The DO deficit is at its minimum regardless of the wind speed. In contrast, at night, when the water surface is being cooled, the oxygen solubility increases and the DO deficit is maximum. Thus, the most oxygen absorption from the atmosphere occurs during the night. The transfer mechanism at night is aided by penetrative convection. This convective process commences when the water surface undergoes cooling, forming a thin cool surface layer that is heavier than the ambient fluid and consequently tends to sink down in form of negatively buoyant plumes. The turbulence generated by this con-vective instability is likely to be the dominant driving force for gas transfer in sheltered lakes or during low-wind periods.
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