380 
BULLETIN OF THE BUREAU OF FISHERIES. 
The following (Table 15) are examples of the determinations of plankton taken 
from different depths in August and September at one of the deepest points of the 
upper Mississippi River, opposite the mouth of the Wisconsin River. The increase 
of volume at depths of from 3 to 6 m., observed on August 14, was due to the greater 
abundance of organisms, not to the increase of detritus. The results of other obser- 
vations concerning the vertical distribution made during the course of the investigation 
are given in Table 29. 
Table 15. — Vertical distribution of planlcton at stations 53 and 143. 
Depth, meters. 
Plankton (cubic 
centimeters per 
cubic meter). 
Depth, meters. 
Plankton (cubic 
centimeters per 
cubic meter). 
Station 
53, 
Aug. 14. 
Station 
143, 
Sept. 14. 
Station 
53, 
Aug. 14. 
Station 
143, 
Sept. 14. 
0 
16 
20 
4.6 
32 
15 
1.5. 
17 
17 
6.1 
36 
15 
26 
17 
1 
7.6 
28 
15 
LAKE KEOKUK. 
There is more plankton in Lake Keokuk than in the adjacent parts of the river. 
In July the mean volume of plankton in the lake, calculated as the average of 143 
samples collected at 30 different stations, was 7.25 cm. 3 per cubic meter. As to the 
richness in plankton there was a marked difference between the upper and the 
lower parts of the lake. The mean volume of plankton in the upper part, between 
Burlington and Nauvoo, was 5.28 cm. 3 per cubic meter; in the lower part, 7.7 cm. 3 
The total volume of the material in suspension observed in the river above Bur- 
lington (station 8) was 3 cm. 3 per cubic meter; 9 miles downstream in the upper part 
of the lake (station 16) it was 4.25. Moreover, the upper part of the lake was con- 
siderably richer in plankton than was the river, because in the lake a great part 
of the detritus is replaced by live organisms. In this case volumetric observations 
are inadequate to determine accurately the increase in the production of plankton, 
and the gradual changes occurring in the river as it widens into a large lake can be 
recognized only by enumeration of the organisms. The distribution of plankton in 
the lower part of the lake was almost uniform (see fig. 11); the volume of plankton 
per cubic meter varied here from 6 to 9.6 cm. 3 There was no increase of plankton 
from Nauvoo down to the dam. In the shallow parts the vertical distribution of the 
plankton was uniform. Near the dam, where the lake is deepest, the amount of 
plankton was considerably greater in the upper strata than in the depths. An 
example of the distribution of plankton as found on July 30 at stations 40, 41, and 
45 follows: 
Table 16. — Vertical distribution of plankton at stations, 41, 45, and 40. 
Depth, meters. 
Plankton (cubic centimeters 
per cubic meter). 
Depth, meters. 
Plankton (cubic centimeters 
per cubic meter). 
Station 
41. 
Station 
45. 
Station 
40. 
Station 
41. 
Station 
45. 
Station 
40. 
0 . 
14 
12 
10 
61 
g 
5 
8 
1.5 
10 
10 
8 
7.6 
7 
6 
C 1 ) 
3 0 
10 
7 
5 
4.6 
6 
7 
10 
10.7 
C 1 ) 
6 
1 Bottom. 
