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 wnutilized 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) asin 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). 
