195 



degree that it has in the older river and lake waters. Its bur- 

 den of silt thus adds to the sources of fertility of the main 

 stream and of the reservoir backwaters at times of flood. The 

 small amount of free ammonia in Quiver Lake (.165) is corre- 

 lated with the small amounts of the substances above named 

 in its waters and the sandy nature of its drainage basin. The 

 differences in the two streams in the quantity of free ammonia 

 (.86 and .245) have the same trend as the differences in plank- 

 ton production (1.91 and .384), but they are not commensurate 

 quantitatively, owing apparently to the more recent origin of 

 the water in Spoon River. In the lakes the free ammonia (.165 

 and .422) and plankton (1.62 and 6.68) exhibit a similar trend 

 and a like absence of quantitative differences in the plankton 

 commensurate with the free ammonia available for support of 

 the plankton. The effect of the relative food supply is thus 

 apparent in the trend of the differences, and the operation of 

 other factors is suggested by the quantitative contrast. The 

 factors in Quiver Lake tending to reduce the plankton below 

 the amount that the food supply would make possible are to be 

 found in the passage of tributary waters through the lake and 

 in the excessive aquatic vegetation. It is noticeable that the 

 considerable amount of submerged vegetation in Quiver Lake 

 does not seem to effect any appreciable increase in the free 

 ammonia. The abundance of free ammonia in the Illinois 

 River would seem to afford a basis for a greater development 

 of the phytoplankton than it attains under the conditions in 

 that stream. The time for breeding which is afforded in the 

 backwaters is one factor involved in this contrast. 



The nitrites constitute a second intermediate stage in the 

 oxidation of nitrogenous substances into inorganic products. 

 Their presence indicates organic matter in the final stages of 

 decay, and that decompositions due to the vital processes of 

 living organisms are under way. The nitrites exhibit a distri- 

 bution in the four localities which in the trend of the differ- 

 ences is similar to that of the free ammonia. The ratio of the 

 free ammonia in Spoon River to that in the Illinois is 1 to 3.4, 



