Summer Stagnation 



47 



months there is no free oxygen at the bottom of the 

 lake, and during August there is not enough oxygen 

 below the ten meter level to keep a fish alive. 



Correspondingly, the amount of free C0 2 in the 

 deeper strata of the lake increases rather steadily until 

 the autumnal overturn. It is removed from circulation, 

 and in so far as it is out of the reach of effective light, 

 it is unavailable for plant food. 



FIG. 8. Dissolved oxygen at different depths in Lake Mendota in 1906. The 

 vertical spaces represent cubic centimeters of gas per liter of water and 

 the figures attached to the curves indicate the depths in meters. (Birge 

 and Juday.) 



Other gases A number of other gases are more or 

 less constantly present in the water; nitrogen, as 

 above stated, being absorbed from the air, methane 

 (CH 4 ), and other hydrocarbons, and hydrogen sulphide 

 (H 2 S), etc., being formed in certain processes of decom- 



