1282 



BULLETIN OK THE BUREAU OF FISHERIES. 



Thus the vital conditions in one part of the lake very sharply limit the possi- 

 bilities of life in another portion. None of those fish which demand a refuge 

 in the cold bottom water during the summer can live in Lake Mendota, nor is 

 it possible to find there those members of the plankton which belong only in 

 the deeper and colder water. 



It is perhaps unfortunate for some reasons that Lake Mendota was necessarily 

 the lake on which the most numerous observations were made, since this lake 

 offers an extreme case in the matter of loss of oxygen from the lower water. 

 The oxygen disappears more quickly and fully than in any other Wisconsin lake 



Of 



I 



2 

 3 

 4 

 5 

 6 



7 



8 



9 



10 



II 



12 



15 



14 



15 



16 



17 



18 



19 



20 



21 



22 



c 



6 4 2 



T Cb 



2 4 6 8 10 lg 14 16 18 20 gg 24 gb gfl 30 3g 34 36 







Cb 



Fig. 8. — Lake Mendota, July 2. 1906. 



of approximately equal size and depth. The area of the lake is so large that the 

 bottom water has necessarily a relatively high temperature, and the plankton 

 is so abundant that the lower water is continually receiving great quantities of 

 decomposable matter. Moreover, the thermocline lies comparatively deep 

 because of the large size of the lake, so that the volume of the cooler water is 

 correspondingly reduced. All of these causes combine to make the disappearance 

 of the oxygen very complete and unusually rapid. This lake therefore affords 

 less opportunity than do other bodies of water for the study of the process in its 

 details. 



