360 



relation in 22 out of 31 instances, and 4 of the 9 exceptions fall 

 in the period of low water in July, during predominance of veg- 

 etation in the lake. The monthly averages in the lake also 

 exceed those in the stream in all months but July and Sep- 

 tember. Higher levels, increased impounding function, and 

 decrease in vegetation thus favor plankton production in Quiv- 

 er Lake, and tend to raise it from a diluent to a source of imme- 

 diate enrichment. 



In this connection it should be noted that the increased 

 production of this year (2.59) still falls below that of the river 

 in 1894 and 1897, and, as seen in the table on p. 292, below the 

 general average of the river production (2.71) ; and also that the 

 higher river levels of this year tend to lower the proportion 

 which the tributary spring and creek waters form of the total 

 volume of Quiver Lake. 



A second significant fact brought out by the comparison is 

 rendered patent by the frequency in this year of the collections 

 in Quiver Lake. The weekly interval from April to Septem- 

 ber (Table V.) makes it possible to trace somewhat fully the 

 movement of production, and demonstrates in Quiver Lake a 

 pulse-like movement in produetion similar to that previously de- 

 scribed in the Illinois River, and one, moreover, ivhich exhibits a 

 very stril-iny coincidence of develojjinental successio)L A superpo- 

 sition of Plate XXVII. upon Plate X. will make this demon- 

 stration apparent. There are exceptions, but these, as shown 

 in the preceding discussion, are in most, if not all, instances to 

 be correlated with local environmental factors confined to one 

 or the other body of w^ater. The return to parallelism with 

 the cessation of the peculiar factor incident to the interruption 

 serves still further to emphasize the significance of this simi- 

 larity. The key to the parallelism must lie in fundamental 

 factors common to the plankton of both areas or to their envi- 

 ronment. 



