i28o 



BULLETIN OF THE BUREAU OF FISHERIES. 



and free carbon dioxide begins to appear there. This process is accentuated as 

 summer approaches. (Fig. 7 and 8.) The amount of free carbon dioxide 

 increases, the thickness of the stratum of the water whose reaction is acid 

 increases also, and the oxygen steadily and rapidly declines in the lower water. 

 By the early part of July the permanent summer conditions of temperature are 

 found. (Fig. 8.) The regular summer thermocline is found lying, in general, 

 between 5 meters and 10 meters. Oxygen has disappeared wholly from the bot- 

 tom waters, and is rapidly going from 





 I 

 2 



3 



A 



5 



6 



7 



6 



9 



10 



II 



I? 



13 



14 



15 



16 



17 



18 



19 



20 



21 



22 



83 



C 

 2 



a 



T 



2 4 







6 a 



10 I? 14 16 



N 34 36 3a 



18 2022 



-I 



C T 



all parts of the lake below the ther- 

 mocline. The water has divided into 

 an upper warm stratum containing 

 an abundance of oxygen and with an 

 alkaline reaction, and a lower, colder 

 layer with free carbon dioxide, and 

 little or no oxygen except in the ex- 

 treme upper part. 



LATE SUMMER AND AUTUMN. 



By the first of August this con- 

 dition has reached its maximum. 

 (Fig. 9). The lake contains no oxy- 

 gen below a depth of 10 meters. 

 Above that level, in the warm water 

 and in the uppermost part of the ther- 

 mocline , there is abundance of oxygen 

 for animal life. Beneath that depth 

 no active animal life is found in the 

 water," though the inhabitants of the 

 mud remain alive through this period, 

 some of them in an inactive condi- 

 tion and some in a partially active 

 state . The carbonates show the char- 

 acteristic summer condition, in which 

 the upper water contains a smaller amount than the lower, the transition coming 

 in rapidly at the thermocline. This condition persists through August and early 

 September for a period varying with the warmth of the season. As the tem- 

 perature of the water begins to fall, the thermocline moves downward under 

 the action of the wind, and this process increases the extent of circulating water 

 and in like degree the thickness of the layer containing oxygen. Figure 10 

 illustrates this condition in early October. The cold weather and winds which 

 are apt to occur at about this time soon bring about a complete mixture of the 



Cb 



Fig. s. — Lake Mendota, April 8, 1906. 



" Except Corethra larvae, whose presence is an apparent rather than a real exception. 



