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 1X., Pl. 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 em.’ 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 em.*, was on Aug. 26, the 
date of the last collection. 
The hydrographic conditions were such (Pl. XLI.) 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 159 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 
em.’ 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. XXXVIII.), Quiver (Pl. XXVIII.), Dogfish (XX XII.), and 
