294 ARCH E. COLE 



Birge ('07), investigating the inland lakes of Wisconsin, dis- 

 covered that in certain of the deeper lakes there was a time 

 each year when the lower water was entirely devoid of oxygen. 

 This stagnation period he found to be a regular phase in the 

 'respiration cycle' of the lake, and he showed that it was brought 

 about largely by physical factors. 



Lake Mendota, which Birge studied with the greatest care, 

 may be taken as an example. In the fall of the year, when the 

 temperature of the lake has become nearly uniform at all depths, 

 winds blowing across the lake set up currents which thoroughly 

 'mix' the water and equalize the amounts of dissolved gases from 

 top to bottom. When the ice covers the lake the mixing ceases 

 and the water becomes more or less thermally stratified. Toward 

 spring, especially in the deeper parts of the lake, the water 

 becomes stagnant, the oxygen being slowly used up by animals 

 and decaying organic material. An excess of carbon dioxide 

 accumulates in the place of the oxygen (Birge and Juday, '11). 

 In the spring, when the ice leaves the lake, the wmd by circulating 

 the water again equalizes the amounts of dissolved gases and 

 makes the lake inhabitable at all depths for aquatic animals. 



As the season advances, the water near the surface of the lake 

 warms most rapidly and a stratification again takes place. The 

 upper water, becoming warmer and hence lighter, floats upon 

 the lower, colder water. The wind is unable to force the warmer 

 surface water down into the colder strata below, and conse- 

 quently a sort of superficial circulation of the upper stratum of 

 water results. The lower water, cut off from contact with the 

 atmosphere, becomes stagnated and loses its oxygen. Three 

 regions are thus formed in late summer and early autumn; 

 the hypolimnion, or stratum of stagnation and low temperature; 

 the epilimnion, or stratum of circulation and higher temperature 

 (Birge, '03), and between these, the narrow thermocline (Birge, 

 '03), or mesolimnion, characterized by a rapid transition in 

 temperature. 



During the summer stagnation period, most of the animals 

 in Lake Mendota migrate from the hypolimnion to the epilimnion. 

 This was shown to be true for fish by Pearse and Achtenberg ('20), 



