410 
BULLETIN OF THE BUREAU OF FISHERIES. 
where they constituted almost 95 per cent of the total number of Cladocera, than in 
the upper, where they were rather scarce. The other species, such as Moina 
macrocopa S., Diaphanosoma leucJiteribergianum F., Bosmina longirostris O. F. M., 
Sida crystallina O. F. M., and Leptodora Icindtii L., were scarce. 
The plankton of Lake Keokuk, in comparison with that of Lake Pepin, is 
characterized by the abundance of Rotifera and by the presence of Pedalion mirurn 
H., which does not occur in other parts of the river; the blue-green algae are less 
abundant here than in Lake Pepin; Cladocera are more numerous in Lake Keokuk, 
and are represented mainly by Moina rectirostris L. and M. bracJiiata J., whereas 
Daplinia retrocurva F., so common in Lake Pepin, is absent in Lake Keokuk. 
At the rise of water the plankton in Lake Keokuk is almost entirely washed 
away, and the difference between the river and the lake with regard to the amount 
and the composition of plankton disappears. This can be seen in Table 28. 
Table 28 . — Plankton of the Mississippi and Lake Keokuk, September 20 to 23. 
[©, very abundant; Q, abundant; ©, frequent; O, scarce; O, very scarce; ..., absent.] 
Volume, cubic centimeters per cubic meter of water 
Species: 
Clathrocystis aeruginosa H 
Melosira crenulata K 
Stephanodiscus niagarse E 
Synedra delicatissima W. S 
Fragilaria crotonensis K 
Pediastrum duplex M 
Codonella lacustris E 
Triarthralongiseta E 
Polyarthra euryptera W 
Copepoaa 
Cladocera 
Lemna 
Detritus 
1 Individuals per cubic meter of water. 
Station 
153, Miss. 
River, 
near New 
Boston, 
Sept. 20. 
Station 
155, Lake 
Keokuk 
at Dallas 
City, 
Sept. 22. 
Station 
156, Lake 
Keokuk 
near the 
dam, 
Sept. 23. 
Station 
157, Miss. 
River, at 
Alexan- 
dria, Mo. 
6.3 
6.2 
4.0 
3.0 
6 
8 
6 
6 
8 
8 
5 
6 
6 
§ 
6 
o 
6 
112 
© 
0 
© 
• 
9 
© 
TRIBUTARIES. 
The tributaries of the Mississippi River carry less plankton than the main 
stream. Only the Skunk River is an exception, and the plankton content of its 
waters emptying into the upper part of Lake Keokuk is from 16.5 to 27.5 cm. 3 per 
cubic meter, or about four or six times more than the plankton content in the 
adjacent part of the lake. The plankton of this river consists almost exclusively 
of Rotifera. Its composition is as follows : 
Stations 13 and 14, Skunk River, July 20, 1921: 
Asplanchna amphora H @ 
Brachionus pala E © 
pala amphiceros E © 
pala dorcas 0 
palaspinosus © 
angularis G © 
1 Individuals per cubic meter. 
Stations 13 and 14, Skunk River, July 20, 1921 — 
Continued. 
Brachionus angularis caudatus B. and D.. © 
Notops brachionus R © 
Clycops (young) 1 10 
Detritus © 
