414 
BULLETIN OF THE BUREAU OF FISHERIES. 
Swift currents are, of course, unfavorable for the development of zooplankton. 
In comparison with the velocity of current the movements of plankton animals 
are so slow that they are unable to obtain a sufficient quantity of food where the 
current is swift. The plankton algae, feeding on salts and gases dissolved in water, 
are in more favorable condition. When a filament of Melosira or a band of 
Fragilaria is drifting many miles with a stream, the processes of assimilation and 
photosynthesis going on in its cells are not interrupted. That is why the algae 
form the larger part of the potamoplankton. 
We have seen that the amount of plankton decreases in swift-running water. 
Thus, below Rock Island Rapids the river carries only about 0.4 as much plankton 
as above the rapids. (Table 27, p. 405, stations 50 and 51) . If we suppose the average 
rate of flow in the rapids to be 3 feet per second, or 1 mile in 29 minutes, it would 
require about 8 hours to pass the rapids, drifting with the current. In this time 
a greater part of the plankton disappears. It may be assumed that the plankton 
organisms are destroyed, not directly by the water running with a great velocity, 
but by the friction against the particles of sand that are suspended in the water. 
Then it becomes probable that the devastating influence of the rapids depends 
not only on the velocity of the stream, but also on the character and the degree 
of roughness of the river bed. Unfortunately we have no direct observations to 
that effect made at different parts of the rapids, and the question remains open. 
It is certain, however, that the amount of detritus is greater below the rapids than 
above, whereas the reverse is the case with the amount of plankton. 
When the water becomes stagnant, or at least flows slowly, the plankton crus- 
taceans grow more numerous. This has been observed in both Lake Pepin and 
Lake Keokuk. The increase of Copepoda and Cladocera is especially noticeable in 
the backwaters of Lake Keokuk, where the crustacean population progressively 
increases from the upper part of the lake to the dam (Fig. 15). 
The stagnant water of Lake Pepin apparently affords more favorable condi- 
tions for Copepoda, which form a considerable part of the plankton of this lake. 
The number of copepods in the river plankton below Lake Pepin, from Reads 
Landing down to Prairie du Chien, is as great as in the lake itself, but above Lake 
Pepin they are less abundant. The water fleas {Daphnia retrocurva and Leptodora 
Icindtii ) also occur in the section of the river between Lake Pepin and Prairie du 
Chien, but their quantities progressively decline from the lake down to Prairie 
du Chien, and below this point these organisms have not been noticed at all. It 
is interesting to note that only adult Leptodora and Daphnia retrocurva occurred 
below Lake Pepin, while both above and in the lake adult as well as young organ- 
isms occurred frequently. 
Daphnia retrocurva is a very variable organism, the various forms differing 
from one another in the shape and length of the head. In Lake Pepin this species 
was represented mainly by forms with an extremely extended crest of the head 
(D. retrocurva proper). In the river below the lake only forms with a short and 
straight crest have been found. 
The plankton of Lake Keokuk, as shown above, differs quantitatively as well as 
qualitatively from that of Lake Pepin. The mean content of plankton in the water 
of Lake Pepin is 2.3 times greater than that in Lake Keokuk. The comparative 
