174 Wisconsin Academy of Sciences, Arts, and Letters. 



sun only very slightly. In Green Lake below 40 or 45 meters 

 the temperature never rises above 6.11 degrees Centigrade, 

 although the surface may run as high as 26.67 degrees C. In 

 Lake Michigan the bottom temperature at depths of 360 feet is 

 4.2 degrees C, v^^ith a surface temperature of 18.3 C. 



The change in the temperature from the top to the bottom is 

 not a gradually decreasing one, however. A layer of water at 

 the surface, which may be in midsummer some ten or twelve 

 meters in depth, is very nearly uniform in temperature. From 

 the lower surface of this layer there is a very rapid decrease in 

 temperature for a short distance, and then a gradual decrease 

 until a minimum is reached. This layer of sudden change in 

 temperature is known as the ^^thermocline," and its position 

 varies in depth with the season and the size of the lake. As the 

 summer season progresses the thermocline grows lower. In the 

 very shallow lakes the temperature during the summer season 

 is nearly uniform through the whole depth. In Lake Winne- 

 bago, for example,- there is seldom a difference between top and! 

 bottom temperatures greater than two degrees. In small lakes 

 the thermocline is considerably higher than in large lakes. This 

 is doubtless due to the influence of the winds, by which the small 

 lake is less affected. This was very prettily illustrated in a com- 

 parison of the V/aupaca lakes with Cedar Lake ancT Green Lake 

 about August 1st of this last summer. In three of the Waupaca 

 lakes — Rainbow, McCrossen and Beasley's, of which Hainbow 

 is the largest and Beasley's the smallest, the thermocline was re- 

 spectively at six meters, five meters and three meters. At the 

 same time the thermocline of Cedar Lake was at eight meters, 

 and the thermocline of Green Lake at eleven meters. 



The vertical distribution of the plankton has a very close re- 

 lation to the thermocline, most of the animals being above it. 

 Limnocalanus, DapJinia longireniis and Daphnia pulicaHa, 

 however, are found below the thermoeline, and in some plankton- 

 poor lakes the proportion of the other organisms below the ther- 

 mocline is much larger. 



It is evident that the circulation of the water is in the layer 

 above the thermocline, and that below the thermocline there is 

 insufficient oxygenation, and that this bottom layer must, too, 



