LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
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8. The plankton of the river is subject to great fluctuations, depending on 
the stage of water. During the rise of the water the plankton is replaced almost 
entirely by detritus and silt. In Lake Keokuk, at the rise of the river, the plankton 
is washed away and plankton samples taken during this period contain detritus 
almost exclusively. 
9. The composition of the plankton of the river may be described as mono- 
tonic. The plankton of Lake Pepin and Lake Keokuk, as compared with that of 
adjacent parts of the river, may be characterized as richer in Crustacea and Rotifera. 
There has been no organism found in the river plankton that could not be found 
in the lakes. 
10. At low-water stages the production of plankton in both Lake Pepin and 
Lake Keokuk is greater than in the adjacent parts of the river. 
11. The increase in the production of fishes in Lake Keokuk since the erection 
of the Keokuk Dam, as recorded by the official statistics, can be correlated with 
the increased production of plankton in this lake. 
