213 
river. Therise in chlorine during the minimum period, July 
to November, indicates the entrance into this lake of sewage- 
laden waters of the river during this period, but it brings with 
it no corresponding increase in the residual nitrogenous sub- 
stances. The depression of the nitrates, and possibly of the other 
forms of nitrogen, may be referred here as elsewhere to their 
utilization by the phytoplankton and submerged vegetation of 
the lake during their period of growth. As in the Illinois River 
and Quiver Lake, the spring maximum of the plankton appears 
at the close of the maximum of nitrogenous substances and is 
followed by their minimum period. The autumn maximum 
appears, at least in 1897, somewhat before any marked in- 
crease in the residual nitrates, though in both this year and 
the following one it extends into the period of rising ni- 
trates. A general correlation thus exists between the seasonal 
production of plankton and the seasonal fluctuations of ni- 
trogenous substances. 
The seasonal fluctuation of the several nitrogenous substances 
exhibits some interrelations with the changes in the plankton, 
and especially with the accession of flood waters, and some 
variations from the general maximum-minimum cycle above 
discussed which call for brief notice. 
The nitrates, the final products of decomposition, exhibit the 
maximum-minimum cycle most clearly, as, for example, in Pl. 
XLV.,XLIX., and L. The fluctuations which affect the other 
substances appear here in diminished prominence, as may best 
be seen by comparing the plottings of Spoon River (Pl. XLVI. and 
XLVI.) with those of Thompson’s Lake (Pl. L.). The close of 
the maximum period of nitrates is usually later than that of 
the free ammonia (Pl. L.), and extends for a varying distance 
into the period of growth of vegetation. This growth in our 
latitude becomes marked in the last days of April and the first 
of May, and continues, in some plants at least, until the frosts 
of October. The nitrates do not reach minimum levels, how- 
ever, (see Pl. XLV.-L.) until late in June. In like manner the 
close of the minimum period is frequently delayed beyond the 
