412 
BULLETIN OF THE BUREAU OF FISHERIES. 
the Viatka River in the Volga Basin (Russia) carries exclusively zooplankton. It 
has been shown in the present investigation that the plankton of the Skunk River in 
July was composed of Rotifera only, no algse being present. We do not know, 
however, that this composition is permanent. The very characteristic peculiarities 
of river plankton are the inconstancy of its composition and a great proportion of 
mineral particles, silt, and various kinds of detritus. 
It has been pointed out by many investigators (Kofoid, 1908; Allen, 1920; and 
others) that river plankton is subject to extreme fluctuations in quantity and 
constitution. Therefore the data concerning the production of plankton obtained 
in different rivers are comparable one with another only when they represent the 
results of long-continued observations. 
The more stable conditions obtaining during low water stages afford an oppor- 
tunity for more abundant development of plankton organisms. Every rise of the 
water level and the consequent increase in the velocity of the current is accompanied 
by a decrease in the plankton population, which is washed away. The river waters 
that contain rich plankton mingle with barren storm water. At the same time 
new forms from ponds, lakes, and other basins connected at the high stage with the 
main stream are brought into the main channel. Therefore the river plankton may 
be characterized as a polymixic plankton with a great proportion of littoral and 
benthal forms. 
The question of the origin of the river plankton has often been discussed in 
scientific literature. Schiitt (1892), on the ground of his observations on the 
Amazon, pointed out that the plankton organisms of that river come from the upper 
tributaries and have not been developed in the main stream itself. Other investi- 
gators — for example, Schmidle (1898) — thought that the river plankton develop in 
the slow-running parts of the stream, in the bayous, and in the lakes forming parts 
of or otherwise connected with the main river. Kofoid (1908), as a result of his 
long-continued observations on the Illinois River, came to the conclusion “that the 
plankton of the channel is not immediately derived from the tributaries, but comes 
in large part from the impounding backwaters, and at low-water stages is almost 
exclusively indigenous in the channel itself.” 
We have seen in the foregoing sections that in the upper Mississippi the total 
amount of plankton is greater in Lake Pepin and in Lake Keokuk than in the 
adjacent parts of the river. We can not say, however, that in all parts of the river 
the plankton is poorer than in the lakes. Thus, for instance, the amount of plankton 
observed on September 12 opposite the mouth of the Root River (station 135) 
— 30.3 cm. 3 per cubic meter — was more than twice as great as 70 miles above, just 
below Lake Pepin, where the average amount of plankton on September 10 at 
Reads Landing (station 124) was only 14.6 cm. 3 per cubic meter. The observations 
made in the river above Lake Pepin show also that there is a considerable increase 
of plankton in the main channel from Hastings down to Red Wing. On September 
2 the amount of plankton near Hastings (station 116) was 12.3 cm. 3 per cubic meter, 
and on September 1, 21 miles downstream near Red Wing (station 110), the amount 
of plankton was 21.5 cm. 3 . At the intermediate station (111) 9 miles above Red 
Wing the amount of plankton observed on the same day was even greater, reaching 
22.7 cm. 3 per cubic meter. 
