324 



ANIMALS IN INLAND WATERS 



temperature. Such convection currents, however, go down only to a 

 certain depth ; below this the water is warmed only by means of con- 

 duction, which acts very slowly. Consequently, below the level to 

 which the daily convection currents extend, a rapid decrease in tem- 

 perature sets in. This level is designated as the thermocline, or 

 metalimnion; the water above it is the epilimnion, and that below it 

 to the lake bottom, the hypolimnion. The thermocline, of course, exists 

 only during the summer stagnation; with the beginning of autumnal 

 circulation it sinks deeper and deeper and at last disappears. This 



4 6 



Temperature in ° C 



8 10 12 14 16 18 20 22 24 



Temperature in ° C 

 6 8 10 12 14 16 18 20 22 



20 40 60 80 100 

 0- Content in % of Saturation 



110 



2 4 



Temperature in ° C 

 6 8 10 12 14 16 18 2 22 24 



20 40 60 80 



0- Content in % of Saturation 



20 40 60 80 100 



0- Content in % of Saturation 



110 



Fig. 93.— Temperature curves ( ), and curves of oxygen content (. . . . .) 



in 



% of saturation, for Cayuga Lake, New York, Madue Lake, Pomerania, and 

 Keller Lake in Holstein. After Thienemann. 



establishes an important classification of lakes on the basis of depth. 

 The location of the thermocline differs in different lakes: in the Mans- 

 feld Lake, in July, it lies at a depth of 1-3 m.; 6 in Lake Sakrow near 

 Potsdam (Fig. 100) it lies at 6-8 m.; 7 in Lake Constance (Fig. 99) 

 between 16 and 20 m.; 8 in Lake Madue (Fig. 93) between 18 and 

 22 m., 9 and at a similar depth in Lake Cayuga; in Lake Mendota in 

 1906, between 7 and 14 m. 



This type of temperature stratification applies to the open water 

 of deeper lakes. In the shore regions of such waters, and in shallow 

 lakes, especially, if they are exposed to mixing by means of the wind, 

 the temperature is more equally distributed through their depths; thus 

 the greatest difference between the temperature of the surface water 



