THE LIFE OF DEVILS LAKE 



31 



The lowering of the oxj-gen in the lower levels is due, not to 

 the absence or scarcity of chlorophyl-bearing organisms at these 

 depths, where they are about as numerous as they are near the 

 surface, but rather to the decomposition of the organic matter con- 

 tained in the ooze covering the lake floor. This decomposition 

 evidently occurs even at zero temperature, judging by the diminution 

 of oxygen in the bottom level during the winter. 



Similar, though much less marked differences, occur between the 

 shore water and that further out in the lake, while local differences 

 may occur, due to aggregations of plants at different points. 



No free carbon dioxide appears in any of the analyses and is 

 probably never present at any point in the lake, though it is pos- 

 sible that at times of high water it might temporarily appear at 

 the mouth of the sewer from Devils Lake city, which discharges 

 into the lake. In the form of bound (mono-) and half bound (bi-) 

 carbonates it usually shows but little difference between surface and 

 bottom, and the shore and middle parts of the lake. At the bottom 

 and near shore the bicarbonates are higher than elsewhere. When 

 the ice is melting the surface water is temporarily much lower in 

 carbonate alkalinity than the lower water. With the disappearance 

 of the ice and stirring of the lake by the wind, this difference 

 promptly disappears. (Figure 9.) 



Figure 9. 



Distribution of CO.- in Main Lake at 4.0 m. from 9,24, 13 to 6, 20/14. 

 OrdinatES represent parts pr. million. 



Numerous tests have been made for hydrogen sulphide, but 

 the results are not very conclusive. 



Birge and Juday (I.e.) have suggested the inexactness of the 

 standard methods for the determination of HoS and recently Heath 

 and Lee (1923), as the result of studies based primarily on Devils 

 Lake water, have pointed out the difficulties in its determination, 

 due to the presence of various salts and their reaction with iodine. 

 These authors suggest the substitution of a colorimetric method for 

 the iodimetric one. 



