196 



while that of the nitrite content of the two streams is 1 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 Illinois 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, .384, and 1.62) in the other local- 

 ities, where the nitrites are but a little less or even greater (.147, 

 .039, and .023). Either 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 zoo- 

 plankton feeding upon it is utilized by some macroscopic animal, 

 — as, for example, by Polyodon, or by the Unionidce 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 



