304 
an immediate dilution of the channel plankton of the Illinois, 
there is a season when its plankton content exceeds that of the 
river. The average of the collections in September—-Novem- 
ber in Quiver Lake exceed by 70 per cent. the average of those 
in the Illinois. This is the season when some autumnal decay 
of vegetation takes place, and this vegetation-rich lake has a 
larger plankton production than the river waters which it 
thus enriches. 
1895. 
(Table V.; Pl. XXVL., XLVIII.) 
There are 13 collections in this year, with an average of .78 
cm." per m.* as compared with 3.22 in the Illinois. The maxi- 
mum of 4.57 occurs on April 29, being but 1.26, or 22 per cent. 
less than the corresponding vernal maximum in the adjacent, 
but—owing to river levels in this season—non-contiguous, river. 
The similarity ia the movement of production between this 
lake and the river noted in the previous year can be traced in 
1895 in but two -instances,—in the rise to the vernal maximum 
and in the increased production in December (cf. Pl. XX VI. and 
IX.). Outside of these periods there is no resemblance between 
the planktographs of the two waters. From July to November 
inclusive the low level of production is broken only by two 
pulses, both of which attend a rise in river levels with increase 
in the impounding function of the lake. These changes in level 
shift the loosely attached vegetation, and are often followed 
by death and decay of masses of aquatic growths. The slight 
rise in the last week in August (Pl. XX VI.) caused an invasion 
of muddy river water into the lower end of the lake. Decay of 
the vegetation and death of many fish, clams, and other ani- 
mals ensued in the invaded area. The flood early in Septem- 
ber (Pl. XXVI.) came largely from up-river rainfall, and the 
lake waters, enriched by invasion, were impounded with result- 
ing increase in the plankton. It was not apparent that either 
of the large collections were made in invading waters, and I 
infer that the plankton was indigenous and not adventitious, 
