216 
times they exhibit indications of a common movement with 
the nitrates or the free ammonia. 
In Spoon River (Pl. 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 with the May maxi- 
mum of the plankton in 1898 (Pl. XLVIITI.). 
In Thompson’s Lake (PI. 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 albuminoid ammonia and the total organic nitrogen 
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. XLUI-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 years 
