193 



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 the several 

 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 



