213 



river. The rise 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 PI. 

 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 (PI. XLVI. and 

 XLVII.) with those of Thompson's Lake (PI. L.). The close of 

 the maximum period of nitrates is usually later than that of 

 the free ammonia (PI. 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 PI. XLV.-L.) until late in June. In like manner the 

 close of the minimum period is frequently delayed beyond the 



