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The thermal bar is a downwelling plume of fresh water at the temperature of maximum density (Tmd), roughly 4o C, or 39o F, that forms in temperate freshwater lakes due to the different rates at which shallow and deep parts of the lake heat up in spring and cool down in fall. Besides seasonal changes in the amount of solar radiation, inputs of river water may contribute to the formation of thermal bars, which roughly follow the shoreline of the lake, and mark the transition between surface waters that are colder than Tmd and water that is warmer than Tmd. As the lake continues heating or cooling, the thermal bar moves towards the center of the lake, disappearing when the entire surface of the lake is either colder or warmer than Tmd. While they are temporary seasonal features, thermal bars play an important role in lake ecology during the time they exist by restricting mixing between coastal and offshore waters, thereby retaining nutrients as well as pollutants near shore, contributing to both increased biological production in coastal waters as well as the concentration and movement of pollutants along the shoreline .
 The effect on lake ecology of climate changes including warmer lake temperatures and and decreased ice cover that change the timing, duration and intensity of thermal bars are unknown.
 

References

1. S. S. Zilitinkevich, K. D. Kreiman, and A. Yu. Terzhevik, “The Thermal Bar,” Journal of Fluid Mechanics 236, no. 1 (1992): 27-42.

2. F. Peeters and R. Kipfer, “Currents in Stratified Water Bodies 1: Density-Driven Flows,” in Encyclopedia of Inland Waters (Oxford: Academic Press, 2009), 530-538.

3. G. K. Rodgers, "The thermal bar in Lake Ontario, spring 1965 and winter 1965-66. In Proc. 9th Conf Great Lakes Res., pp. 369-374. University of Michigan, Great Lakes Res. Div., Publ. 15.

4. R. A Moll et al., “Physical, Chemical, and Biological Conditions Associated with the Early Stages of the Lake Michigan Vernal Thermal Front,” Estuaries 16, no. 1 (1993): 92–103.

5. Paul R. Holland and Anthony Kay, “A review of the physics and ecological implications of the thermal bar circulation,” Limnologica - Ecology and Management of Inland Waters 33, no. 3 (September 2003): 153-162.