350 
BULLETIN OF THE BUREAU OF FISHERIES. 
desmium collect as water bloom in certain areas and not in others, and that diatoms 
near the edge of the polar ice occur in more or less local swarms. 
According to Gran the irregularities just mentioned do not invalidate the 
general statement but arise because the conditions of existence vary even in closely 
adjoining areas. The distribution of zooplankton is more irregular. The organisms 
may gather in certain areas in abundance and maybe scarce in an adjacent area. 
This frequently occurs in the sea alongside currents and in bays. Moore, Edie, 
Whitley, and Dakin (1912), criticizing Putter’s ideas, have shown that in a com- 
paratively small area of sea surface there may be no uniformity in distribution of 
the plankton. We may admit that the plankton is uniformly distributed in a 
definite area of sea or lake if the conditions of existence in said area are uniform, 
but this does not mean that there is a uniform distribution of plankton in the 
whole basin. When we are studying the productiveness of a definite part of sea, 
lake, or pond we are chiefly interested in finding out the quantity of organisms 
living in the whole body of water. As to their horizontal and vertical distribution 
these are quite different problems. 
Many observations have been made on American and European lakes and 
ponds showing the irregularities in distribution of plankton (Huitfeld-Kaas, 1906; 
Skorikow, 1905; Galtsoff, 1914; Moberg, 1918). Bruno Hofer (1896), investigating 
the Bodensee Lake, came to the conclusion that the distribution of plankton may be 
called uniform when the difference between the volumes of plankton taken in two 
different parts of the lake does not exceed 25 per cent. The productive capacity 
of different parts of a lake may be different. Such a condition prevails when the 
form of the lake is irregular and there are many areas of shallow water covered with 
water plants. Thus Skorikow observed that the productiveness of Lake Pestovo 
(Russia) is greater near the banks and in shallow waters than in the pelagic region. 
Even in the pelagic region of a small and rounded lake the distribution of the plank- 
ton may be very irregular. In some cases observed by the writer in Kossino Lake 
near Moscow, Russia (Galtsoff, 1914), the difference in the volume of plankton in 
different parts observed simultaneously was more than 400 per cent. These condi- 
tions, however, were not stable and would change within 24 hours, the wind and 
the current apparently being the cause of such irregular distribution. 
The estimation of the productive capacity of the basin must be based on many 
observations. A small number of catches obtained from the pelagic region is in- 
sufficient, and the conclusions drawn from such observations may be erroneous. The 
present investigation embodies a comparative study of the plankton of Lake 
Keokuk, Lake Pepin, and the Mississippi River between these two lakes, and is based 
on the examination of a large number of samples taken in various parts of the river 
and in the lakes. Some observations also were made in St. Croix Lake and in other 
tributaries of the Mississippi. All observations were made during the period from 
July 10 to September 24, 1921. 
The author desires to express his gratitude to Dr. R. E. Coker, formerly in charge 
of the division of scientific inquiry, United States Bureau of Fisheries, for his many 
suggestions concerning the investigation; to R. L. Barney, director, and to H. L. 
Canfield, superintendent, United States Fisheries Biological Station, Fairport, Iowa, 
who so materially aided and facilitated the field investigation; to C. A. Sears, man- 
