422 



Flag (XXXIV.) lakes by 50 to 150 per cent., — is to be attributed 

 to the diluent effect of invading flood-waters from Spoon River, 

 whose plankton content on Apr. 27 was only .05 cm. 3 per m. 3 

 The entrance of these flood waters, indicated by the check in 

 the decline of the hydrograph (PI. XLL), was noticeable at the 

 station on the day of collection, and is the cause of the increased 

 turbidity in Phelps Lake on that day (Table IX.). 



The similarity in the movement of production in Phelps 

 Lake in 1897 to that in the other bodies of water examined by 

 us, is very close. In the case of the Illinois River, 8 out 9 pos- 

 sible instances, or 89 per cent., are in agreement ; in Thomp- 

 son's Lake 7 out of 9, or 78 per cent.; in Flag Lake all in- 

 stances are in .agreement ; in Quiver Lake 6 out of 9, or 67 per 

 cent. ; and in Dogfish Lake 4 out of 6, or 67 per cent. As a whole, 

 32 out of 40, or 80 per cent., of the changes in the direction of 

 production in Phelps Lake accompany similar changes in di- 

 rection in these other localities. All of the 8 exceptions to this 

 agreement occur at levels below 8 ft., when local environments 

 are more potent, and 5 of the 8 are found in Quiver and Dogfish 

 lakes, where vegetation and access of tributary waters become 

 proportionately more or less potent as levels fall or rise in May 

 to August, when the 5 exceptions occur. 



This unusual degree of agreement in 1897 must be attrib- 

 uted in large part to the hydrographic conditions in the period 

 of comparison. For almost 5 months of the year levels were 

 above 8ft., when fluctuations have relatively but a slight effect 

 on the various environments. Above this level the several lo- 

 calities are more or less submerged in the general overflow, and 

 all share alike in the wide stretches of open water in which some 

 current exists, and the commingling to an increasing extent, 

 as levels rise, equalizes and obliterates local differences in pro- 

 duction. The first 4 collections of this year were made under 

 such conditions, and agree without exception in the course of 

 production. The remaining 5 were taken at stages below 8 ft., 

 in the very midst of the season of local diversification, and the 

 proportion of agreements falls from 100 to 60 per cent., and is 



