38 



Nature of Aquatic Environment 



final coalescence in October. Leaving aside the not 

 unusual erratic features of surface temperature (repre- 

 sented by the topmost contour line) it will be noticed 

 that there is a wider interval somewhere between 8 and 

 1 6 meters than any other interval either above or below 

 it. Sometimes it falls across two spaces and is rendered 

 less apparent in the charting by the selection of inter- 

 vals. It first appears clearly in June at the 10-12 meter 

 interval. It rises in July above the 10 meter level. 



Fig. 7. Temperature of the water at different depths in Lake Mendota in 

 1906. The vertical spaces represent degrees Centigrade and the jfigures 

 attached to the curves indicate the depths in meters. (Birge and Juday). 



In the middle of August it lies above the 8 meter level, 

 though it begins to descend later in the month. It 

 continues to descend through September, and is found 

 in early October between 16 and 18 meters. It dis- 

 appears with the beginning of the autumnal circulation. 

 The cause of this phenomenon is not known. Richter 

 has suggested that convection currents caused by the 

 nocturnal cooling of the surface water after hot summer 

 days may be the cause of it. If the surface waters were 



