405 



that in other years, and this also tends to lower the relative 

 productiveness in the lake. It is evident that the seasonal dis- 

 tribution of the period of flood v\^aters and the resulting im- 

 pounding function of the lake affect greatly its contributions 

 to channel plankton. In this year flood waters are largely 

 confined to the colder and less productive season, when the 

 run-off contains little plankton and its contributions are small, 

 while in 1897 recurrent floods throughout the year afforded a 

 run-off in seasons of larger production, and this tended to 

 greatly increase the enrichment of channel waters in that year 

 as compared with 1897. 



Plankton content in 1897 in Thompson's Lake was in ex- 

 cess of that in the river in the case of coincident or approxi- 

 mate collections in 16 of the 18 instances, the two exceptions 

 appearing in July and September, when pulses in channel plank- 

 ton rise above the recorded production in the lake as a result 

 of some undetermined factor. 



The similarity in the course of plankton production here 

 and elsewhere is most marked in the first part of the year, 

 and decreases in the time of low water. Thus, on comparison 

 of the planktographs of Thompson's Lake (PI. XXXVIII.) and 

 the Illinois River (PI. XI.) w^e find 14 out of 18 changes in the 

 course of production coincident in the two regions, the four ex- 

 ceptions occurring in May (1), July (2), and September (1). 

 The environmental differences between Thompson's Lake and 

 the river are much less than between this lake and Quiver, and 

 we find a corresponding disagreement in their planktographs, 

 only 10 out of 18 changes being in the same direction, and six 

 of the ten are in the period of high water, w^hen local differen- 

 ces are submerged. In the cases of Flag and Dogfish lakes col- 

 lections extend only to July, with agreement in 5 cases in each 

 out of a possible 7 and 6 respectively. In the year as a whole 

 and including all the above localities we find 34 agreements to 

 15 exceptions, in January-June the ratio being 3 to 21 for 4 lo- 

 calities, andinJuly-December, in low water conditions, 12 to 13 

 for from 2 to 3 localities. The effect of the common elements 



