LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
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single form present in plankton of the main channel of the river that could not be 
found in the lakes or in the tributaries. 
The plankton consisted principally of diatoms and blue-green algse. These 
two groups together in most samples made up more than 75 per cent of the total 
mass. Next to them were Chlorophycese, which occurred in almost every sample, 
and Rotifera, which were especially abundant in the upper part of Lake Keokuk. 
The Copepoda were also present in abundance and were very numerous in Lake 
Pepin. 
Table 24 . — The composition of the plankton of the Mississippi River tributaries. 
[•, very abundant; O, abundant; ©, frequent; ©, scarce; O, very scarce; absent.] 
Among the diatoms, Melosira crenulata (E) K. is the most common form. It 
has been found at all stations on the river and on the lakes except Lake St. Croix, 
where it is wholly replaced by Melosira granulata (E) Ralfs. Another species, 
Fragilaria crotonensis K., is also widely spread. It was very abundant in the upper 
