300 



The decreased volume of water causes a relative concentration 

 of the sewage and consequent increase in fertility of the chan- 

 nel water over that of the usual high water of this season. 

 Lower levels insure more rapid rise in temperature, and the 

 slackened current affords more time for the breeding of the 

 plankton. The occurrence of Moina micrura, a lover of foul 

 water, is in itself an index of the character of the stream in 

 this low-water spring. The contributions of the impounded 

 backwaters to the stream during this April- June period (see 

 PI. IX.), owing to the small areas submerged, are reduced in 

 volume so that both the relative and actual share which they 

 have in the formation of the channel plankton is probably less 

 than in years of normal spring flood. Nevertheless, as seen in 

 Plate XXXVI., such waters as Thompson's Lake tend by their 

 run-off to enrich and increase the channel plankton. 



The month of July (PI. IX.) witnesses the rapid decline of 

 the second vernal pulse from 29.68 cm. 3 on the 6th to 6.8 on 

 the 23d and .33 on the 29th — a fall of 98 per cent, in 23 days. 

 The last stages in this decline were hastened by the rise of 3 

 ft. in the third week of July, the flushing and destructive action 

 of the flood waters continuing until the close of the month. 



In this and subsequent years I shall call attention — when- 

 ever the interval between collections is brief enough to afford 

 adequate data — to the phenomenon of recurrent pulses of 

 plankton production. I am led to make this emphasis by ob- 

 serving in the numerical analysis of these catches recur- 

 rent pulses in most if not all of the more abundant species, 

 pulses, moreover, which exhibit a degree of concurrence in 

 many species which I believe to be expressed in the faintly 

 traceable volumetric pulses which run like waves, erratic in 

 amplitude but more regular in interval, through the seeming 

 vagaries of the volumetric data. I shall therefore treat the 

 volumetric data from this point of view, endeavoring to discov- 

 er evidence of cyclic production wherever it exists, and seek- 

 ing to correlate this phenomenon with the more patent fac- 

 tors of the environment. 



