LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
387 
Table 18 . — Volumes of plankton collected simultaneously with pump and with vertical plankton net. 
LAKE KEOKUK. 
58.. 
59.. 
63 !: 
64.. 
65.. 
66.. 
67.. 
27.1 
25.5 
20.4 
68. 
72. 
75 
79. 
81. 
82. 
84. 
85. 
17.1 
16.0 
17.1 
15.0 
15.2 
19.5 
15.2 
-34.5 
-12.5 
-26.8 
-7.0 
LAKE ST. CROIX. 
11. 3 28. 2 
12. 2 26. 9 
DISTRIBUTION OF COPEPODA AND CLADOCERA. 
THE RIVER. 
The crustacean population of the river consists principally of copepods (Diap- 
tomus and Cyclops), the Cladocera comprising only an insignificant part of the 
plankton. In that part of the river between Rock Island Rapids and Burlington 
Crustacea are scarce. (See Table 19.) In this region the average number of 
Copepoda in July and August did not exceed 60 individuals per cubic meter, and 
Cladocera were almost entirely absent. In the latter part of September an un- 
expected increase of crustacean population was found in Sturgeon Bay near New 
Boston (station 151), where the number of Copepoda reached 3,520 and that of 
Cladocera 2,170 per cubic meter. In the adjacent parts of the main channel they 
were absent. Ninety per cent of the water fleas found in Sturgeon Bay was repre- 
sented by Moina rectirostris, and Diaphanosoma leuchteribergianum formed the 
remainder. In July the number of crustaceans at the same locality was 60 Copepoda 
and 20 Cladocera. 
Above Rock Island Rapids the number of crustaceans gradually increased up- 
stream (Table 19) ; near Prairie du Chien the number of Copepoda reached 29,000 
per cubic meter (September 14) ; between Prairie du Chien and Reads Landing 
