426 



immediate operation during the last half of the year in so far 

 as Phelps Lake is concerned, and other factors common to the 

 whole environment or inherent in the common plankton must 

 be responsible for the similarity in this period. 



1899. 



(Table IX., PI. XLII.) 



There are but 6 collections in this year, — in January-March, 

 at fortnightly intervals. The hydrographic conditions are such 

 that the lake is cut off from the river for 34 days during the 3 

 months, and of the remaining time there were only 32 days of 

 stages above 10 ft. in which currents passed through the lake to 

 the river, and 7 of falling stages at levels below 10 and above 8 

 ft., when the run-off continued, making a total of 39 days of 

 contribution to channel waters. These times of contribution 

 in January and March (PI. XLII.) are also times of high plank- 

 ton production for that season of the year. Thus the plank- 

 ton content in Phelps Lake on Jan. 24 is 8.47 cm. 3 per m. 3 to .03 

 in channel waters. The run-off from the lake at that time 

 is thus 286-fold richer in plankton than the water it joins. 

 Again, in March, it is 3- to 9-fold greater. The monthly aver- 

 ages of production are (see table following p. 342) from 6- to 

 26-fold greater in the lake than in the river. This lake thus 

 serves, even in winter conditions and under a thick and long- 

 persisting coat of ice, as a rich breeding ground for plankton 

 whose run-off enriches the channel plankton. This is due to 

 its impounding function, which results in high production, as, 

 for example, during the decline of the January flood (PI. XLII.). 

 Proof of this is seen in the sudden decline in production (from 

 9.3 cm. 3 on Feb. 7 to .1 on the 21st) when flood waters from 

 Spoon River were scouring out the lake beneath the ice. 



The movement in production in these months in Phelps 

 Lake bears little resemblance to that elsewhere, agreeing with 

 changes in channel production (PI. XIII.) in only 1 out of 6 

 possible instances, and in 3 and 2 respectively out of 6 in the 

 case of Quiver and Thompson's lakes. This exceptional disa- 



