LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
413 
It follows from these observations that a considerable difference in the produc- 
tion of plankton sometimes occurs within a comparatively short distance, which may 
not exceed 10 or 15 miles. Therefore the increase of plankton in some parts of the 
main channel of the Mississippi is due to the greater productive capacity of that part 
of the river, not to the mixing with the waters of its tributaries. 
This can be seen clearly from the following : The plankton content of St. Croix 
River, as compared with that of the Mississippi River, is very high, averaging 29.2 
cm. 3 per cubic meter, whereas the amount of plankton of the Mississippi River 
observed simultaneously at station 116, just above the mouth of the St. Croix 
River, is only 12.3 cm. 3 per cubic meter. The amount of plankton of the Mississippi 
River at station 115, 1 mile below the mouth of the St. Croix River, is 16.3 cm. 3 , or 
only 4 cm. 3 greater than above the mouth, whereas 10 miles downstream the content 
of plankton increases to 22.7 cm. 3 per cubic meter. There are no other tributaries 
in this section of the river and the increase of plankton evidently is due to a greater 
productive capacity of this part of the main river. 
There arises a question : What is the cause of the higher or lower productivity 
of the different parts of the river ? The writer is unable to answer this satisfactorily 
because the solution of this problem requires many special local investigations and 
observations as to the chemical composition of water, which could not be made 
during the course of the present investigation. 
It has been observed by many investigators that the amount of plankton in 
the river depends mainly on the hydrographic conditions, and especially on the 
velocity of current. Allen (1920), on the basis of a statistical study of the plank- 
ton of the San Joaquin River, Calif., came to the conclusion that “water currents 
above a very moderate speed are distinctly inimical to plankton development.” 
The same idea has been expressed more precisely by Schroder (1899) in his paper 
on the phytoplankton of the Oder River (Germany). He says that the amount 
of plankton in the running water of the river is in inverse proportion to the slope 
of the river. In Steuer’s textbooks on limnology this statement is called “ Schroder’s 
law” (Steuer, 1910, p. 107). There is no sufficient reason for designating as a 
“law” such a statement, which is made mainly to describe the phenomenon and 
which can be applied only to a limited number of cases. 
The amount of plankton in a given part of the river depends not only on the 
slope of the river, and consequently on the velocity of the current, but also on 
the hydrographic conditions in the upper parts of the river. We have seen that 
the slope of the Mississippi River above Rock Island Rapids (0.35 foot per mile) 
is almost the same as below the rapids (0.38 foot per mile). The distance, 16 
miles, where the river passes through the rapids with a total fall of 21 feet, divides 
the river into two sections, distinct from each other in their plankton content. 
The rich plankton of the upper part of the river, averaging 21.3 cm. 3 per cubic 
meter, is evidently destroyed in the rapids, as the plankton content in the lower 
part of the river averages only 5.16 cm. 3 , although from Davenport, just below 
the rapids, to Burlington, about 100 miles down, the slope is the same as in the 
upper section of the river, its plankton resource is not restored, and an increase 
in plankton occurs only in the backwaters of Lake Keokuk. 
