384 
BULLETIN OF THE BUREAU OF FISHERIES'. 
The figures on Figure 12 indicate the average volume of plankton in cubic 
centimeters per 1 cubic meter calculated for each station from the data obtained 
by pump collections. The curves are drawn between the points of equal content 
of plankton. The difference in the amount of plankton in each of the two parts 
of the lake is presented on this map in a vivid manner. The maximum volume 
of plankton, 25.8 and 27 cm. 3 , is found in midlake in the lower part; the minimum 
amount, 7.5 cm. 3 , is found in the northern shallow part of the lake near Bay City. 
In the shallow water of the lower part of the lake between Pepin Village and 
the Chippewa Delta the amount of plankton is very great, ranging from 20 to 27 
cm. 3 per cubic meter. This area is covered with water plants, Potamogeton crispus, 
P. americanus, Vallisneria spiralis, and Ruppia occidentalis, forming large asso- 
ciations. 
It is interesting to compare the content of plankton in the water running 
into the lake with that in the outflow. For this purpose we may use the observa- 
tions made August 29 at stations 96 to 98, 1 mile above the head of the lake, and 
August 30 at stations 101 to 103, opposite Reads Landing, below the foot of the 
lake. The average amount of plankton above the lake was 16.6 cm. 3 per cubic 
meter, while that in the outflow was 21.8 cm. 3 The increase in plankton in the lake 
then was 5.2 cm. 3 per cubic meter, or 31.3 per cent. It is noteworthy that the 
increase in plankton occurred only in the lower part of the lake (see fig. 12) . The 
mean amount of plankton in the upper part, excluding the northern shallow section, 
as has been shown, averaged 15.7 cm. 3 , or 0.9 cm. 3 less than that in the Mississippi 
above the lake. 
The vertical distribution of plankton in Lake Pepin is the same as in Lake 
Keokuk. On calm days the surface water sometimes contained more plankton 
than the lower layers, but after the wind had been blowing for a long time the 
whole body of water would be stirred up, and the distribution of plankton would 
become uniform. The results of the observations are presented in Table 29 (p. 426, 
stations 58 to 95). The uniformity of the distribution of plankton in a lake, so 
far as its volume is concerned, depends principally on the uniformity of temperature 
distribution. Of course the different organisms may be distributed differently 
in the vertical line, and, even if the amounts of plankton in the upper stratum 
and at depths are the same, they may consist of different components. In Lake 
Pepin the greatest difference in the distribution of plankton was due to the blue- 
green algae, which were most abundant at the surface of the lake and very scarce 
in the depths. 
LAKE ST. CROIX. 
The average volume of plankton observed at Lake St. Croix September 2 
was 29.3 cm. 3 The observation was made on a calm morning. The vertical 
distribution of plankton at three stations located across the lake is shown in Table 17. 
Apparently the decrease in plankton occurs on the levels where the fall of tempera- 
ture is the greatest (7.6 to 9.1 m., station 112; and 6.1 to 7. 6 m., station 113). 
