limnological observations IN THE UPPER MISSISSIPPI. 
391 
Table 20 . — Mean content of plankton and mean numbers of Copepoda and Cladocera in cross sections of 
the lower part of Lake Keokuk, July 15 to 30, 1921. 
Station number. 
Distance 
from the 
dam, in 
miles. 
Copepoda, 
mean num- 
ber per 
cubic 
meter. 
Cladocera, 
mean num- 
ber per 
cubic 
meter. 
Mean vol- 
ume of 
plankton, 
cubic cen- 
timeters 
per cubic 
meter. 
23 to 25 
11. 5 
2,000 
2,400 
8.3 
26 to 28 
2, 300 
1,900 
7.6 
29 to 31 
6! 5 
3,600 
1,500 
8.9 
32 to 34 
4.5 
2,600 
1,300 
6.4 
35 to 37 
3.5 
3,900 
600 
7.5 
38 to 40 
2.5 
13, 600 
2,600 
8.2 
41 to 43 
1.6 
7,100 
8, 100 
7.6 
44 to 46 
0.5 
8,200 
3,400 
7.1 
/ 
Fig. 14.— Distribution of Cladocera in Lake 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.) 
The vertical distribution of plankton Crustacea in Lake Keokuk varied greatly, 
as is evident fromTable 29 (p.422), in which the numbers of Copepoda and Cladocera 
observed at different depths are given for each station. These organisms can 
gather in great abundance at any depth from the bottom to the surface of the 
lake. The difference in the vertical distribution was sometimes very great even 
