197 



grosser forms of submerged aquatic vegetation, it is likewise 

 effectually removed from the field of analysis until again 

 released by the decomposition of this vegetation. The nitro- 

 gen as ammonia in organic compounds, or as nitrites, is either 

 entirely unavailable for plants or, with the probable exception 

 of the free ammonia and the amido-compounds, is less availa- 

 ble than the nitrates. These other forms consequently more 

 fully represent the potential fertility of the water than the ni- 

 trates do, for the latter indicate mainly the unutilized portion 

 of the nitrogenous plant food immediately available. In the 

 light of the foregoing conditions more significance attaches to 

 the distribution of nitrates and plankton in the four localities. 

 The excess in the river (1.58) over that in the tributary waters 

 of Spoon River (1.01) and Quiver Lake (.66) may be due in part 

 to the greater age of the waters of the main stream and the 

 opportunity thus afforded for the completion of the processes 

 of decomposition of organic substances delivered to the main 

 stream by tributaries above the point of examination. When 

 the quantity of nitrates in the river is compared with the or- 

 ganic nitrogen, free ammonia, nitrites, and nitrates in Spoon 

 River or Quiver Lake, it becomes apparent that the tributary 

 waters of this stream still act as a diluent of the river water. 

 The source of this excess in the main stream is to be found in 

 the sewage and industrial wastes of Chicago and Peoria. The 

 unutilized nitrates are two and a half times as great in the 

 river (1.58) as in Thompson's Lake (.64). In so far as the ni- 

 trates are concerned, both Spoon River and the Illinois might 

 support a much more abundant plankton than they now pro- 

 duce (1.91 and .384) if the conditions permitted. Thompson's 

 Lake, drawing its water from these sources, does maintain a 

 greater production (6.68) and exhibits a great reduction in the 

 amount of nitrates (.64), the unutilized residium being less 

 in this lake than in any of the other localities. The increase in 

 the amount of plankton in Thompson's Lake over that in the 

 river (3.5 times as much) is roughly proportional to the decrease 

 in nitrates in the lake as compared with the river (.4 as much)'. 



