16 



Indiana University Studies 



Mr. G. N. Hoffer made ten bacterial counts in these lakes. 

 In all but two, the counts showed that the bacteria are more 

 numerous at the surface, at the bottom, and at a point usually 

 in the thermocline, than at other levels. While the data are not 

 elaborate enough to warrant wide generalizations, they appear to 

 substantiate the postulate of Juday concerning the decrease of 

 oxygen in the thermocline; and the suggestion that I have just 

 made, namely, that considerable plankton disintegrates at or 

 near the surface. The bacteria counts are appended. 



The rate of reduction of the amount of oxygen in the 

 hypolimnion depends upon the amount of organic matter present 

 and the various factors that influence the date of thermal strati- 

 fication. As I have indicated, the latter are of two sorts, a group 

 that is constant for any given lake and a group that is variable. 

 The former group includes dimensions and shape of basin, relation 

 of long axis to direction of prevailing winds, the surrounding 

 topography, etc. The latter includes the weather conditions 

 previous to the stratification. If the weather is warm during this 

 period, with winds of low velocity, stratification occurs before the 

 temperature of the lake is raised very much so that the water in 

 the hypolimnion remains cold. 



If, however, the weather is cool and windy, the water of the 

 lake continues to circulate and the temperature of the whole 

 lake rises. When stratification is finally established, the tempera- 

 ture of the hypolimnion is much higher than if the weather had. 

 turned suddenly warm. 



The whole interrelation of factors that infiuence the reduction 

 of oxygen in the hypolimnion in lakes is so complex that it cannot 

 always be inferred from a summer series. 



In Center lake, for instance, the temperature at the bottom 

 in 1912 was 49° while in 1913 it was 55°. The water certainly 

 circulated for a longer period in 1913 than in 1912. In 1912 there 

 was more oxygen at 7 meters than at 6 meters in 1913. In both 

 years there was no oxygen in the lower levels. 



In Turkey lake the bottom (18 M.) had a temperature of 60° 

 and oxygen was still present, while Little Eagle (Chapman) 

 had no oxygen at the bottom (11 M.) with practically the same 

 temperature. This difference is probably due to the greater 

 growth of plants in Chapman which furnished an abundant 

 supply of oxidizable material. 



In Webster lake there are several rather deep basins connected 



