32 



Nature of Aquatic Environment 



every clear day with the going down of the sun, but 

 great changes, important enough to affect the tempera- 

 ture of all the waters of a deep lake, occur but twice a 

 year, and they follow the precession of the equinoxes. 

 There is a brief, often interrupted, period (in March 

 in the latitude of Ithaca) after the ice has gone out, while 

 the surface waters are being warmed to o°C.; and 

 there is a longer period in autumn, while they are being 

 cooled to o°C. Between times, the deeper waters of 



W/NTER 



SUMMER 



Fig. 4. Diagram illustrating summer and winter temperature conditions in 



Cayuga Lake. The spacing of the horizontal lines represents 



equal temperature intervals. 



a lake are at rest, and they are regularly stratified 

 according to their density. 



In deep freshwater lakes the bottom temperature 

 remains through the year constantly near the point of 

 maximum density, 4° C. This is due to gravity. The 

 heavier water settles, the lighter, rises to the top. 

 Were gravity alone involved the gradations of tempera- 

 ture from bottom to top would doubtless be perfectly 

 regular and uniform at like depths from shore to shore. 

 But springs of ground water and cvirrents come in to 



