432 



to stable hydrographic conditions, abrupt changes being limited 

 to less than 8 weeks in the year (PL IX.), and to low water 

 and consequent slackening in the channel with increase of time 



PLANKTON PRODUCTION IN 1895* 



Station 



Illinois River. 

 Quiver Lake. . 

 Dogfish Lake. 

 Flag Lake. . . 



Thompson's Lake . 



Feb. 



o.oi — 

 0.03 - 



Apr. 



31 



3.00 



8.20 



May 



l-|- 28.20 + 61 .44 — 



June 



+ 



30.42 

 o. 12 



9.42 + 



July 



9-33 

 0.37 

 2.99 



4.83 



Aug. 



4.03 

 0.21 

 I . II 



3-09 



Station 



Sept. 



Illinois River 



Quiver Lake 



Dogfish Lake 



Flag Lake 



Thompson's Lake. 



1.52 

 0.94 

 3-15 



3.58 



Oct. 



+ 



0.57 

 o. 13 

 0.52 

 57-76 

 3-15 



Nov. 



+ 



3.02 

 0.05 

 501 

 14.40 

 5.07 



Dec. 



+ 



1. 14 

 0.46 

 5-32 

 4.82 

 1 .00 



Mean of 



monthly 



Averages 



+ 5-91 

 0.65 



330 

 25.66 



13-31 



+ 



*The minus sign signifies below average and the plus sign above. 



for the breeding of the plankton. The larger amounts of plank- 

 ton were found only during stable conditions, and floods inva- 

 riably depleted the volume of the plankton. These periods of 

 stable conditions occur in summer and late autumn, and we 

 find the plankton content at such times 3- to 30- fold that in 

 contiguous flood conditions. In the river the monthly produc- 

 tion exceeds the monthly average for our records in 5 out of 

 the 9 months represented, the exceptions being February, when 

 stagnation under the ice prevailed, April, a vernal period of 

 low water without overflow, September, a month of repeated 

 floods, and October, when an unusually early decline in temper- 

 ature occurs. 



The stable conditions which attend low water thus favor 

 the increase in the plankton content per m.^ though by reason 

 of the lower levels and slackened current the total volume pro- 

 duced in the stream as a whole must be greatly diminished by 

 such hydrographic conditions. 



The results of this low-water year upon production in Quiv- 

 er Lake (PI. XXX.) are as a whole diametrically opposite to 

 those in the river. Here in the lake, production falls below 



