GASES IN WATERS OF WISCONSIN LAKES. 



I281 



water (fig. 11); and with it returns the condition of uniformity which we found 

 at the opening of winter. From this time until the lake freezes the temperature 

 declines almost uniformly at all depths ; the amount of oxygen increases as the 

 capacity of the water to hold it rises with the fall in temperature; and the 

 water becomes almost or quite neutral as the vigor of the growing algae 

 declines and as decomposition becomes slower in the cooling water. 



From this brief account it is plain that the cycle of gas changes in Lake 

 Mendota is a very important factor in determining the conditions and possibilities 

 of life in that lake. Here is an inland lake, one of the largest in Wisconsin, 



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II 



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IS 



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about 9 kilometers in length and 6 kilometers in breadth, with a maximum 

 depth of 24 meters and an average depth of about 12 meters, the lower half 

 of whose water is wholly uninhabitable during middle and late summer and 

 early fall. This water in the early spring is saturated with oxygen, as is the 

 lower water of all lakes, but the supply is not great enough to meet the demands 

 which are made upon it. The lake is peculiarly rich in plankton, and the 

 decomposition of the great amount of animal and vegetable debris which is 

 showered down from the upper waters into the lower soon exhausts the oxygen 

 supply and renders the lower water unfit for the maintenance of higher life. 



