Temperature fine-structure of Lake Michigan hypolimnion 1 , 2

High-resolution temperature profiles from the hypolimnior of Lake Michigan in June and August 1978 showed the frequent occurrence of intrusive layers of thickness 10 m or less. Anomalously warm layers ( Delta T 0.1 degree C) seemed to extend horizontally for 0.5 km and to persist at one location for day. Since temperatures were above maximum density values, these layers would have been hydrostatically unstable were it not for observed stabilizing gradients of dissolved silica. Interleaving of the layers, shear-induced turbulence, and stirring at the bottom resulted in various small-scale temperature structures. After an intense coastal upwelling event, a bottom layer 5-m thick was observed along the 100-m isobath in which temperatures and silica concentrations were nearly uniform and in which a value of at least 0.1 crn super(2).s super(-1) was calculated for the vertical eddy diffusivity.