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, 
