LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
415 
poorness of Lake Keokuk in plankton is due to several causes. First, the water 
of the Mississippi River running into Lake Keokuk carries less plankton than the 
water of the inflow into Lake Pepin. Second, the hydrographic conditions are 
more unstable in Lake Keokuk than in Lake Pepin, both because of the operation of 
the dam, which causes fluctuations in the water level, and because of the fact that 
although both lakes are interpolated in the course of the river, Lake Keokuk is more 
intimately connected with the river than Lake Pepin. Due to the delta that the 
Mississippi River has built in the northern upper part of Lake Pepin, the inflow 
of the river into the lake is limited to a relatively narrow canal, while in Lake 
Keokuk the whole body of the river water is held in check and the river is pro- 
gressively transformed into the lake. Therefore, every change of the river condi- 
tions immediately affects the whole body of water in the lake. 
In each of the two lakes the conditions surrounding the plankton and other 
ecological communities may differ from that in a typical lake. The most important 
peculiarity consists in a constant renewal of water which affects, of course, the whole 
organic life in these so-called “ river lakes.” The last term has been proposed by 
Coker (1914). It very clearly expresses the profound difference that exists between 
a typical lake and such a body of relatively still water as is intimately connected 
with the river and where the water is constantly renewed. 
A sudden decrease of plankton may occur in the river lakes, where, due to the 
rise of the water level, the whole plankton resource can be washed away and replaced 
by silt and detritus. Such a case has been observed in Lake Keokuk when, at the 
end of September, the rise of the river caused the total disappearance of relatively 
rich plankton developed in the lake during the low stage. It is probable that Lake 
Pepin is affected in a smaller degree by the changes in the river stage, and therefore 
affords more favorable conditions for plankton development. The present observa- 
tions on Lake Pepin were completed on the 10th of September when the river was 
still at a low stage, and therefore this question remains open. 
Obviously the complete cycle of life in the “river lakes,” the plankton pulses, 
the appearance and disappearance of plankton forms, the seasonal fluctuations in 
the amount and composition of plankton, and even the distribution of plankton 
and bottom organisms is different from that in typical lakes. Lake Pepin and Lake 
Keokuk afford a rare opportunity for making a comparative study of the life in 
two river lakes, one of which is a natural lake, the other an artificial lake not as yet 
completely developed. One could expect that the study of the organic life in these 
two lakes carried on during the whole year should solve many interesting problems 
concerning the biology of the river plankton and its relation to the plankton of the 
river lakes. 
PLANKTON AND THE FISHERIES. 
Many attempts have been made to use the results of the quantitative investi- 
gations of plankton as a basis for estimating the productiveness of ponds or lakes 
in fishes. From a practical point of view the problem is of great importance, espe- 
cially in connection with the artificial propagation of fishes. The question as to 
whether there is a sufficient quantity of natural fish food in the pond in which the 
fishes are to be raised is the first one to be answered by the fish-culturist before stock- 
