216 



times they exhibit indications of a common movement with 

 the nitrates or the free ammonia. 



In Spoon River (PI. XLVI. and XLVII.) the summer rise 

 in nitrites is not apparent except in the low water of 1897. 

 The decay of organic matter is thus less active during this 

 season in tributary water than it is in the main stream. In 

 contrast with the summer, the winter exhibits somewhat more 

 nitrites, but these are not markedly different in amount from 

 those in the main stream at that season. The only correlation 

 between the nitrites and the plankton of this stream appears 

 in 1897 from May to December, when plankton maxima are 

 uniformly attended by decrease in nitrites. As elsewhere, they 

 present no constant relation to the fluctuations of other forms 

 of nitrogen. 



In Quiver Lake (PL XLVIII. and XLIX. ) the nitrites have 

 their maximum during the colder months and the flood period. 

 A marked depression of nitrites appears Avith the May maxi- 

 mum of the plankton in 1898 (PL XLVIII.). 



In Thompson's Lake ( PL L. ) the changes in the nitrites 

 are slight, irregular, and without apparent correlation either 

 with other nitrogenous substances or with the plankton. Like 

 the nitrates, the nitrites are not greatly and immediately 

 affected by the accession of flood waters, and they run lower 

 in the reservoir backwaters than in the main stream. 



The alhuiitinoid anntionia and the total organic nitroyen 

 fluctuate together so closely (see PL XLIX.) that it seems un- 

 necessary to distinguish between them in this discussion. The 

 seasonal fluctuations in these substances in the Illinois River 

 (PL XLIII.-XLV.) are not marked, as a result apparently of 

 the somewhat uniform accession of sewage. The dilution of 

 the sewage consequent upon overflow is to some extent offset 

 by the large accessions of these substances, which as silt and 

 leachings accompany flood waters. A slight increase attends 

 concentration in the low water of 1897 (PL XLIV.), and a 

 slight decrease comes with the period of overflow of the same 

 year. Similar movements are less evident in the other yearis 



