LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
349 
formation of animal and plant comr mnities. Of course, we should not forget the 
peculiar situation existing in Lake Keokuk, where a great quantity of old vegetation 
is now in a state of decomposition; therefore the conditions in the lake are unstable, 
and many changes will take place before the lake fauna and flora will be finally 
fixed. 
The location of the lake as a part of the river is also worth considering, especially 
because there is no marked boundary between the river and the head of the lake; 
the river is gradually transformed into a lake, and every change in the river water 
immediately affects the whole body of the water in the lake. The presence of 
another lake, a natural one, also forming a part of the river and located 450 miles 
upstream and 240 miles to the north by direct line gives us the opportunity to 
compare the organic life of these two lakes one with the other and with that of 
the river. 
The chief problem we have to investigate is how the organic life in the river 
has been affected by the new condition created by the dam and the consequent 
formation of a new lake. The solution of this problem requires many systematic 
and long-continued observations made at all seasons of the year. It is quite 
impossible to solve it completely after a short investigation, as such an investigation 
gives us only the general characteristics of the river and lake and may be used as 
a basis for further detailed study only. 
The present investigation, made in the summer of 1921, is a study of the com- 
position, amount, and distribution of plankton in various parts of the river and 
in the lakes. According to Hensen’s (1887) principal work on plankton, the plankton 
organisms lie at the base of all the life in water. Hensen advanced the idea that 
the plankton is uniformly distributed in the sea and concluded that the determina- 
tion of the amount of plankton under a unit of area of any part of the sea would 
afford a measure of the productive capacity of that part. This idea has been used 
very often as a basis in hydrobiological investigations of inland waters, especially 
for the determination of the productive capacities of ponds used for fish culture. 
Among the water organisms the planktonic forms are the most sensitive to the 
external conditions of existence. Every change in the surrounding medium affects 
these forms immediately, suppressing the reproduction of some and furthering that 
of the others. Therefore the composition of the plankton is characteristic for 
every type of basin, and its quantity may serve as an indicator of the productive 
capacity of a pond or lake because the number of higher animals, such as fishes 
and mussels, whether permanent inhabitants or only temporary visitors, depends 
directly or indirectly on the quantity of plankton existing in the basin. 
The importance of plankton to other organisms living in water has been 
acknowledged by all scientists, but the question of the regularity of its distribution 
has been very much disputed. An especial interest in this question has arisen since 
Putter’s work (1907) on the nutrition of sea animals by organic matters dissolved 
in water. Hensen (1887), Lohman (1901, 1903, 1908), Gran (1912), and others 
pointed out that the distribution of the pelagic plants in the sea at any rate is 
extremely regular. Lohman has found that at certain seasons 10 to 15 cm. 3 of the 
sea water are sufficient to give a representative sample of the total plankton. At 
the same time Gran has shown that often in tropical waters dense masses of Tricho- 
