GASES IN WATERS OF WISCONSIN LAKES. 



1285 



This lake has an area of about 51 hectares (126 acres) and a depth of about 

 22 meters. It contains an abundance of oxygen in the water above the ther- 

 mocline; then follows a stratum in which the gas has almost or quite disap- 

 peared; then comes one containing a small amount, but sufficient for the 

 maintenance of a large number of plankton animals; and beneath this to the 

 bottom the water contains no oxygen. A little later, in September, all of the 

 oxygen will have disappeared from the lower water, and the region beneath the 

 thermocline will become uninhabitable by animal life. 



LAKES OF NORTHEASTERN WISCONSIN. 



All of these illustrations are taken from the lakes of southeastern Wiscon- 

 sin, where, as shown by the diagrams, the dissolved carbonates are present 

 in large quantities, and where the -, 



plankton life is correspondingly abun- Q 0. T 28 30 32 34 36 



dant. In the lakes of northeastern 

 Wisconsin, where the carbonates are 

 low, the average quantity of plank- 

 ton is much less than in the hard- 

 water lakes. It is not true that the 

 plankton of every soft-water lake is 

 smaller than that of every hard- water 

 lake. The lakes of both types differ 

 among themselves very greatly, but on 

 the average the statement is entirely 

 correct. European observers have 

 found the same thing for the fish in 

 the lakes of Switzerland that we have 

 found for the plankton in Wisconsin. 

 We have also found that there are 

 fewer fish in the northern lakes than 

 in the southern, hard- water lakes. In 

 these northern lakes, therefore, with 

 their poorer plankton, the oxygen per- 

 sists longer than in the southern ones. 

 It may disappear entirely , but it usually 

 lingers late, and in the deeper lakes it 

 is apt to remain throughout the season. 



The diagrams of Thousand Island Lake and Stone Lake (fig. 15, 16) show 

 the summer condition in two of the larger and deeper lakes of this type. The 

 carbonates in both are low, representing about 9 centimeters and 3 cubic 

 centimeters of carbon dioxide, respectively. The upper water is acid, or nearly 

 neutral, the acidity increasing below the thermocline. The oxygen curve of 

 Thousand Island Lake closely resembles that of Green Lake, having a depression 



Fig, 



