We present observations of the evolution of a deep-mixing event in a deep, temperate lake. The observations were obtained from thermistors mounted on a long-term mooring in the lake. The event seems to have originated near 150-m depth and resulted in a plume or layer of cold water from the upper half of the lake that descended to the lake bottom (590 m) over a 3-d period. Net mixing associated with this event resulted in an overall vertical heat exchange of nearly lOI J and a volume exchange of 0.7-3.2 km? (4-18 % of the lake volume) between the upper and lower portions of the lake. The deep water displaced during the event is estimated to have carried 0.3-2.5 X lo6 mol of nitrate to the upper lake, which accounts for a significant portion of the average annual nitrate flux (-2-4 X 10 ” mol yr-‘) thought to be upwelled in this highly oligotrophic system. The hypolimnion of deep, temperate lakes is usually considered to be largely isolated from the surface waters and the atmosphere. In recent years, however, some attention has been focused on hypolimnetic mixing and deep-water formation in such environments. Various measurements in bot
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