196 
while that of the nitrite content of the two streams is | to 3.7. 
The ratios in the two lakes, Quiver and Thompson’s, are 1 to 
2.6 and 1 to 2.1 respectively. Spoon River and Quiver Lake are 
thus poorer in nitrites than [linois River and Thompson’s Lake. 
The same contrasts are to be found in their production of 
plankton, though the differences in the amounts produced are 
greater than those in this source of fertility. The amount of 
nitrites (.048) in Thompson’s Lake is quite, low when the large 
plankton production in this lake (6.68) is contrasted with the 
much smaller amounts (1.91, .884, and 1.62) in the other local- 
ities, where the nitrites are but a little less or even greater (.147, 
.039, and .023). Hither the nitrites are an inadequate measure 
of the potential fertility of the water, or the other waters named 
might, in the environment of Thompson’s Lake, support a more 
abundant plankton. 
The nitrates are the final products of the oxidation of ni- 
trogenous matters, in which the nitrogen returns to inorganic 
compounds and is once more in a form most available for util- 
ization as food for the phytoplankton or other aquatic plants. 
The quantity of these compounds is a prime index of the im- 
mediate fertility of the water, and becomes a basis for future 
growth of the phytoplankton and other aquatic plants. The 
amounts of nitrates present in the waters of the four localities 
are very different, and at first glance exhibit little correlation 
either with the other forms of nitrogen present in the water or 
with the quantity of plankton produced. It should be noted 
in this connection that the nitrates, more completely perhaps 
than any other form of nitrogen, are utilized by the chloro- 
phyll-bearing organisms as food, and if taken up by the phyto- 
plankton the nitrogen appears in the subsequent analysis as 
organic nitrogen. If, however, the phytoplankton or the z06- 
plankton feeding upon it is utilized by some macroscopic animal, 
—as, for example, by Polyodon, or by the Unionide which cover 
the river bottom in places,—it is removed from the field of 
analysis, excepting only in such animal wastes as are returned 
to the water by the feeding organism. If it is utilized by the 
