LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
403 
Other rather common Protozoa are the following: Arcella vulgaris E., Dif- 
jlugia pyriformis P., and Codonella lacustris E. Codonella lacustris, which has a 
great resemblance to Difflugia pyriformis, is more abundant in the lakes, but the 
latter is found more often in the river samples, especially when taken near the 
bottom. This list of the planktonic Protozoa should be longer, but as preserved 
material chiefly has been available for study, many Ciliata and Flagellata could 
not be identified. 
The Rotifera are scarce in the river and become more abundant in the lakes, 
the upper part of Lake Keokuk being especially rich in these organisms. In the 
main channel of the river the most common forms found at nearly every station 
are the following: Anursea cochlearis G., Brachionus pala E., Br. pala amphiceros 
E., Br. angularis G., Br. angularis caudatus B. and D., Diurella stylata E., and 
Polyarthra euryptera W. The same species occur in excess in the lakes where the 
rotifer population is more varied, and some species, absent in the river samples, 
are found in great abundance. 
The plankton crustaceans are very scarce in the lower part of the river and 
more abundant in the upper part and in the lakes. The cladoceran population 
in Lake Pepin is mainly represented by Daphnia retrocurva F. ; that in Lake Keokuk 
by Moina rectirostris L. and M. brachiata J. The relative frequency of these and 
other cladoceran species will be discussed later. 
The plankton of the Mississippi River, in comparison with that of Lake Pepin 
and Lake Keokuk, is characterized by the absence of several forms abundant in 
the lakes, and a great part of its volume is composed of organic and inorganic 
detritus. There has been found no form in the running waters of the Mississippi 
that was not present in the samples taken in the lakes, which is quite natural, 
because both lakes are but reservoirs of the river water, and the plankton organisms 
carried by the latter rapidly multiply and become more abundant in the stagnant 
water of the lakes. 
THE RIVER. 
It has been stated above that with regard to the richness of plankton there 
is a great difference between two main sections of the river, below and above the 
Rock Island Rapids. The river below the rapids, from Davenport to Burlington, 
is very poor in plankton, the samples taken in this section consisting of detritus, 
silt, a little sand, and few organisms. The plankton here was especially poor in 
July. An analysis of some of the samples taken near Fairport on July 11 and 12 i's 
given in Table 25. 
It is noteworthy that the plankton crustaceans, both Copepoda and Cladocera, 
were entirely absent. There was no considerable difference between the main 
channel of the river and the Andalusia Slough. Andalusia Slough is a long and 
shallow lateral channel of the Mississippi, but passable by boat. It is about 10 
miles long, and during the time of observation was from 2 to 3 feet deep. The 
current was slower than in the main channel. One would naturally expect to find 
more organisms in the samples taken in the slough, but the analysis of the July 
sample shows that they are scarcer in the slough than in the main channel. (See 
Table 25.) One month later, however, on August 9, the plankton collected at the 
same localities was generally richer, and in Andalusia Slough it was more abundant 
than in the main channel. The composition of the plankton collected on August 9 
is given in Table 25. 
