THE RESPIRATION OF AN INLAND LAKE 



347 



which is taking place rapidly in the lower water; and not infrequently 

 a far larger amount of organic life may be found in these strata than 

 in any other portion of the lake. This process is necessarily limited 



20 T g5 



Fig. 1. Elkhart Lake, Aug. 23, 1905. The vertical spaces indicate the depth in meters. 

 The horizontal spares indicate cubic centimeters of oxygen per liter (parts per thousand of 

 volume), as shown by the line O---O. They also indicate the temperature in centigrade de- 

 grees, as shown by the line T-—T. The upper, warm layer of water is about 6 m. thick ; the 

 thermocline extends from 6 m. to about 10 m. The presence of the manufactured oxygen is 

 very plainly seen in the space between 5 m. and 12 m.; the amount of oxygen rising to a maxi- 

 mum of over 10 c.c. per liter at the depth of 8 m. The relation of this gas to the temperature 

 of the water and the consequent stratification is clearly shown. 



to lakes whose upper warm layer is thin, and is confined to the upper 

 part of the cold water, since only there can the light have sufficient 

 intensity to carry on the operation. But even thus restricted, it is of 

 great value to some lakes. 



I have said little hitherto of the carbon dioxide — a gas whose impor- 

 tance is quite equal to that of oxygen — and now can only sketch a part 

 of its complex story. This gas plays many roles in the respiration of 

 the lake. It is at once the waste product of the tissue activity of plant 

 and animal, the product or by-product of decomposition, and the indis- 

 pensable food of green plants. The lake may obtain the gas from the 

 air, and to some extent does so. Carbon dioxide exists in the atmos- 

 phere in very small amount — about four parts in 10,000. Minute as 

 this quantity is, the land plants are able to secure from it ample sup- 

 plies of carbon. The movement of the air is so free and such enor- 

 mous quantities pass over the surface of the plants, that they readily 

 pick up the gas in large amounts. But the situation of the algse and 

 other plants of the lake is very different, as they must secure their 



