LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
389 
The number of crustaceans gradually decreased from the head of Lake Pepin 
upstream as far as Hastings. The copepod population at station 116 (September 1) 
near Hastings was 1,053 per cubic meter, that of Cladocera 160, while just above 
Lake Pepin (station 96, August 29) the Copepoda content in the main channel 
reached 7,840 and that of Cladocera 1,270 per cubic meter. 
The distribution of crustaceans across the river was very variable. In many 
cases it was observed that they were more numerous in the main channel than 
close to the banks or in the shallows. (See Table 19, statons 96 to 98 and 129 to 
131. Both Stations 96 and 97 are located on the main channel.) As was shown 
at station 106, however, where the water was almost stagnant, the copepods were 
three times as numerous as in the main channel (station 104). Among the water 
lilies that form large zones along both banks of the river above Prairie du Chien 
crustaceans were very scarce (station 56). 
The number of Copepoda in Lake St. Croix, as observed on September 2, varied 
from 12,460 per cubic meter near the shore to 18,315 in mid lake. The correspond- 
ing figures for Cladocera were 680 and 1,955. 
The crustacean population was very scarce in most of the tributaries of the 
Mississippi River. A considerable quantity of copepods was found only in Zumbro 
River (station 126, September 10, 27,200 per cubic meter), in Black River (station 
132, September 12, 5,960 per cubic meter), and in Wisconsin River (station 142, 
September 14, 2,120 per cubic meter). Only occasional copepods or water fleas 
can be found in the samples collected in other tributaries emptying into the Missis- 
sippi. (See Table 24.) 
LAKE KEOKUK. 
Crustaceans were more abundant in Lake Keokuk than in the adjacent part 
of the river. The distribution of the Copepoda and Cladocera population in the 
lake is presented graphically in Figures 13 and 14, where the figures at each station 
indicate the average number, in hundreds, of organisms per cubic meter of water. 
The lines on the maps are drawn between the points with equal content of organisms. 
Crustacea were scarce in the upper part of the lake between Burlington and 
Nauvoo, where their number at different stations varied from 0 to 400 per cubic 
meter. In the lower part of the lake they were more abundant. Here the average 
number of Copepoda at different stations varied from 600 to 23,500 per cubic meter 
and of Cladocera from 100 to 14,500. The mean number of Copepoda in the lower 
part of the lake, computed as the average of 25 stations, was 5,400; that of Cladocera, 
2,720. The mean number of Crustacea in the upper part of the lake could not be 
computed because of the small number of stations and the very different ecological 
conditions existing here. 
In the lower part of the lake the crustacean population gradually increased from 
a line between Nauvoo and Montrose down toward the dam. This increase can be 
noticed on Figures 13 and 14, but it is more obviously demonstrated in Figure 15 
which represents the mean number of Copepoda and Cladocera at eight cross sections 
of the lake. The ordinates of this figure represent the mean number, in hundreds, 
of individuals at each cross section; the abscissae, the distances in miles between 
the line connecting Nauvoo and Montrose and the dam. The continuous line repre- 
sents the fluctuations in the number of Copepoda; the dotted line, in that of the 
Cladocera. The maximum content of Copepoda occurs about 2 \ miles above the 
