390 
BULLETIN OF THE BUREAU OF FISHERIES. 
dam; that of the Cladocera, at 1 % miles above the dam. Each of the two groups 
shows a decline in the section close to the dam, yet they are more abundant there 
than near Nauvoo. 
The distribution of the Copepoda along the lake is shown on Figure 13. It is 
remarkable that they are more numerous near the shores than in the mid-lake region. 
All stations in the lower part of the lake are located in the pelagic region; therefore 
the differences in the abundance of organisms refer to one ecological area. The 
distribution of Cladocera in this lake is, in general, the same as that of Copepoda 
and differs only in details (fig. 14) . 
Fig. 13. — Distribution of Copepoda in Late Keokuk, July, 1921. (Figures on the chart indicate the average number of indi- 
viduals, in hundreds, per cubic meter of water. Figures beneath the chart correspond to the serial numbers of stations in 
the cross sections; upper figures refer to the left side stations.) 
As has been stated above, the distribution of the total amount of plankton in 
the lower part of Lake Keokuk was almost uniform. The mean quantities of plank- 
ton for each cross section of the lake, computed in the same way as the mean number 
of Crustacea, varied from 6.4 to 8.3 cm. 3 per cubic meter, but there was no increase 
in the lower sections near the dam. The volume of the plankton at the two sections 
nearest to the dam was even less than in the section opposite Nauvoo (Table 20 and 
fig. 15). Evidently Crustacea, which were about five times more numerous in the 
lower sections than in the upper, replaced there some other constituents of the 
plankton. 
