1286 



BULIvETIN OF THE BUREAU OF FISHERIES. 



at the thermocline, and an increase in the deeper waters, followed by a gradual 

 decrease to the bottom, where the oxygen is nearly or quite exhausted. This 

 Thousand Island Lake is one of the lakes in northeastern Wisconsin that contain 



lake trout {Cristivomer namaycush) . 

 The fish inhabits the bottom water 

 during the summer, and seems to be 

 found native only in lakes which carry 

 an abundance of oxygen to the bot- 

 tom. The oxygen curve of Stone 

 Lake (fig. i6) shows another interest- 

 ing fact which is observable in many 

 inland lakes, namely, an increase of 

 oxygen in the upper part of the ther- 

 mocline. This is due to the presence 

 of algae at this depth, which still re- 

 ceive sufficient light to manufacture 

 starch, and liberate oxygen in that 

 process. This phenomenon is very 

 commonly found in our lakes, but by 

 no means universally. The crops of 

 algae, which produce oxygen, are not 

 necessarily continuous ; nor is oxygen 

 produced in quantities beyond con- 

 sumption at all periods of their growth. 

 The thermocline may lie so deep or 

 the water may be so opaque that 

 algae in the cool water do not get 

 light enough to enable them to produce 

 starch. Neighboring lakes, therefore, 

 which seem quite similar, may or 

 may not show this rise of the oxygen. 

 In the same lake it may appear and 

 disappear during the season, and 

 although usually present at a corre- 

 sponding time in successive seasons 

 may vary greatly in amount. One 

 common result of this process is the 

 using up, at this point, of the carbon 

 dioxide, the fact being shown by a reduction of the acidity, or an increase 

 in the alkalinity, of the water. The contrast between the carbon dioxide 

 curves in Thousand Island and Stone lakes is sufficiently noteworthy, and this 

 difference is apparently due to the activity of the algae at a depth of 9 meters 



Fig. 



OCT 



12. — Green Lake, October 4, 1906. 



Cb 



