Upwelling of deep water during thermal stratification onset—A major mechanism of vertical transport in small temperate lakes in spring?

Using airborne thermal infrared imaging and horizontally resolved in situ temperature monitoring at the lake surface, we estimated strength and duration of regular wind-driven upwelling of dense deep water to the lake surface in two small (in terms of Rossby radius) temperate lakes during the initial phase of summer thermal stratification. The onset and duration of the upwelling events correlated well with the balance between stratification (in terms of Schmidt stability) and wind forcing, as expressed by Lake and Wedderburn numbers. The period of regular upwelling appearances lasted 7-15 days, identified by Schmidt stabilities around 30 J m(-2) and Lake numbers between 0 and 1, and resulted in persistent temperature gradients of up to 2 degrees C across the lake surface. Our results suggest that spring upwelling should inevitably take place in all freshwater temperate lakes with mean temperatures crossing the maximum density value of freshwater on annual cycle, whereas duration and intensity of the upwelling would vary depending on lake morphometry and weather conditions. Our results suggest major contribution of upwelling in nutrient supply to the upper waters, oxygenation of the deep water column, and air-lake gas exchange, in particular, the release of the sediment-produced methane into the atmosphere.