396 
BULLETIN OF THE BUREAU OF FISHERIES. 
With regard to the distribution of Copepoda in the different parts of the lake, 
it is interesting to note that they are more numerous in the mid-lake region and less 
abundant near the shores. As we have seen, the reverse condition was found in 
Lake Keokuk. 
The quantity of Cladocera in Lake Pepin was considerably less than that of 
Copepoda, the mean number per cubic meter being only 1,020, or about one twenty- 
fifth that of Copepoda. Each cubic meter of water running into the lake carries 
approximately the same average number of water fleas as can be found in 1 cm. 3 of 
lake water. The number of Cladocera in the outflow was only 230 per cubic meter. 
The maximum quantity of Cladocera, 11,200 per cubic meter, was found in the upper 
part of the lake close to the shore (station 86). The fluctuations in abundance of 
Cladocera in the different parts of the lake, represented in Figure 17 and in Table 21 
show a decrease from the head to the foot of the lake. They are more abundant 
near the shores than in the mid lake. This can easily be seen on Figure 18, which 
is drawn in the same way as previously. 
As compared with Lake Keokuk, Lake Pepin is considerably richer in Crusta- 
cea, especially in Copepoda, the mean number in Lake Pepin being 4.7 times greater 
than in Lake Keokuk. Cladocera, however, are more abundant in Lake Keokuk 
than in Lake Pepin, the mean number being 2.6 times as great in Lake Keokuk as 
in Lake Pepin. 
In each of the lakes copepods are more abundant than cladocerans. The ratio 
between the mean number of Cladocera and the mean number of Copepoda is: 
In Lake Keokuk, 1:2; in Lake Pepin, 1:25. These relations are presented in the 
following table: 
Table 22. — Comparison of Copepoda and Cladocera in Lakes Keokuk and Pepin. 
Mean 
number of 
Cladocera 
per cubic 
meter. 
Mean 
number of 
Copepoda 
per cubic 
meter. 
Ratio of 
Cladocera 
to Cope- 
poda. 
Lake Keokuk 
2,720 
1,020 
h, 400 
1:2 
1:25 
Lake Pepin 
Ratio of Crustacea in Lake Keokuk to that in Lake Pepin: Cladocera, 2.7: 1; Copepoda 1: 4.! 
COMPOSITION OF THE PLANKTON. 
The examination of 673 samples collected by different methods in the river, 
lakes, and mouths of the principal tributaries makes it possible for us to describe 
the composition of the plankton of the upper Mississippi with a certain accuracy. 
One of the problems of the present investigation consists in the comparison of the 
plankton of the river itself with that of Lake Keokuk and Lake Pepin, which form 
parts of the same river. 
