192 



cept in the case of Quiver Lake, where the oxygen consumed 

 (5.9) is proportionately very much lower than the loss on igni- 

 tion (27.5). The amount of oxygen consumed is greatest in 

 Spoon River (14.1), and may be attributed largely to the detri- 

 tus of organic origin which the stream carries, or to the prod- 

 ucts of its decay held in solution. It may also be due in part 

 to the organic material of the water-bloom (Euglena) which es- 

 capes the silk of the plankton net. There is, however, no in- 

 crease in the oxygen consumed in the season of the water-bloom 

 which can be considered commensurate with its development. 

 Nitrogen is an essential constituent of protoplasm and of 

 many of its products. It is taken up by plants in the form of 

 nitrates and free ammonia, and there is increasing evidence 

 that it may be utilized, especially by the lower plants, in more 

 complex combinations, such as the amido-compounds. Since 

 the other principal constituents of protoplasm — carbon, hydro- 

 gen, and oxygen — are present in inexhaustible quantity in the 

 air, water, and carbon dioxid, and since the nitrogen available 

 for plant food is practically limited to that contained in the 

 above-named compounds, the nitrogen in combination in any 

 given body of water becomes par excellence an index of its fer- 

 tility. These compounds exist in living plants and animals, in 

 their wastes, and in the products of their decay. They enter 

 stream and lake waters in various ways : in the debris of veg- 

 etable and animal origin washed into the stream, especially by 

 flood waters ; in leachings from such matters drawn from the 

 soil in seepage and spring waters ; and, especially (in the Illi- 

 nois River) in the sewage and industrial wastes of Chicago, 

 Peoria, and other cities within the drainage basin. In the lake 

 and stream waters these nitrogenous compounds are found in 

 solution in the water, in the sediment and debris of organic or- 

 igin in suspension, in the zoo- and phytoplankton, and in the 

 macroscopic aquatic plants and in the larger animals — such as 

 fish, mollusks, insects, and crustaceans. The chemical anal- 

 yses show only those nitrogenous compounds in solution, in 

 silt, and in plankton, while that stored in the larger plants and 



