421 



a variable factor from which Phelps Lake is to a large extent 

 exempt. The degree of agreements, as a whole, in production 

 between Phelps Lake and other localities is seen in a total 

 of 84 instances out of a possible 141, or 60 per cent. 



1897. 



(Table IX., P). XLI.) 



There are only 9 collections in this year, at approximately 

 monthly intervals with the exception of the last collections, 

 when the interval was somewhat reduced. Collections cease in 

 August, when the water entirely disappeared from the lake. 

 The average plankton content for the year is 10 cm.^ per m.'^ 

 the lowest annual average in which summer collections are in- 

 cluded in this lake. The vernal pulse was not detected, if pres- 

 ent, and the maximum record, 29.94 cm.', was on Aug. 26, the 

 date of the last collection. 



The hydrographic conditions were such (PI. XLL) that a 

 current from the flooded bottom-lands about Spoon River 

 passed through the lake uninterruptedly from Jan. 7 to May 13, 

 and the run-off of impounded waters continued until the 25th, 

 a total of 139 days. Throughout the period covered by our 

 few collections they indicate that the plankton content of this 

 area exceeded that in channel waters by from 1.6- to 11-fold 

 with the single exception of Apr. 27, when the lake had 4.26 

 cm.^ to 5,11 in the channel. During the 139 days of run-off 

 the production in the lake scarcely exceeds 5- fold that in the 

 channel, but when discharge ceases the content rises to 10- to 

 11-fold that in the river — a phenomenon which illustrates the 

 equalizing effect of general overflow on the one hand, and the 

 effect of impounding in increasing production on the other. 



The run-off from this lake in 1897 thus predominantly 

 served to enrich channel plankton. The fact that produc- 

 tion in Phelps Lake falls below that of the channel on Apr. 27, 

 when a vernal pulse might be expected of an amplitude greater 

 in backwaters than in channel,— as indeed it is in Thompson's 

 (PL XXXVIIL), Quiver (PI. XXVIIL), Dogfish (XXXIL), and 



