195 
animals is not determined. Since the silt is undergoing decay, 
and since the individuals of the plankton are short-lived and 
rapidly release their nitrogenous compounds into the water by 
waste and decay, the determinations of nitrogen in its various 
forms in the analyses represent both the present fertility and 
that in immediate prospect. The contributions from the ma- 
croscopic plants and animals not included in the samples 
analyzed constitute an undetermined element in the sum total 
of the nitrogenous matter available for the sustenance of the 
phytoplankton. The relative amounts of nitrogen in theseveral 
stages of decomposition are shown in the determinations of 
total organic nitrogen, of nitrogen as albuminoid and free am- 
monia, and of nitrites and nitrates. 
The total organic nitrogen includes all nitrogen that is in 
combination with carbon (together with other elements) in the 
tissues of living plants and animals and in many of the waste 
products of the latter. It is also present in organic matter in 
the early stages of decay, and is accordingly found in organic 
debris and sewage of stream and lake waters. It is accord- 
ingly an index of the quantity of organic matter which in its 
present form is not available for plant food (with the possible 
exception of certain amido-compounds) but is destined to be- 
come available by decay. It thus indicates the potential fer- 
tility of the water. The differences in the amount of total 
organic nitrogen present in the four localities are not in each 
case correlated with the actual plankton production. Spoon 
River, which contains the least plankton, has the greatest 
amount (1.292) of organic nitrogen. The absence of any ex- 
cessive contamination by sewage in this stream combined with 
the paucity in plankton, makes it apparent that this mat- 
ter is probably in the organic detritus of the silt, which is pres- 
ent in an unusual amount in this stream. The close resem- 
blance of the Illinois River and Thompson’s Lake in the matter 
of total organic nitrogen (1.03 and and 1.05) is explained by the 
dependence of the latter upon the river for its water supply, 
and by the excess of sewage in the former and of plankton in 
