LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
411 
The composition of the plankton in other tributaries is given in Table 24. It 
is evident from this table that, in comparison with other rivers emptying into the 
Mississippi, Zumbro and Black Rivers are the richest in plankton. The number 
of Copepoda observed in the mouth of Zumbro River reached a considerable figure — 
23,200 per cubic meter — exceeding even the content of Copepoda in the adjacent 
part of the Mississippi River, where their number was 14,500 per cubic meter 
(station 127) . Leptodora Irindtii and Daphnia retrocurva occurred also in those rivers. 
Swift streams, such as the Chippewa and Wisconsin Rivers, are very poor in 
plankton. The material suspended in their waters consisted almost exclusively of 
sand and detritus which was deposited immediately below their mouths, for min g 
large sand bars in the Mississippi River. 
The observations in other rivers, except Edwards River, were made during 
the rise of the water at the end of September, and therefore their results are not 
comparable with those at a low stage. The samples taken from these rivers con- 
tained nothing but silt, sand, and occasional Melosira and Codonella. 
DISCUSSION AND CONCLUSION. 
POTAMOPLANKTON. 
As can be seen from the foregoing sections, the plankton of the upper Mississippi 
River is mainly composed of diatoms, blue-green algae, and Rotifera. This agrees 
with the results of many other observations made of different American and Euro- 
pean rivers; the river plankton or, using Zacharias’s term, potamoplankton, is 
generally composed mainly of diatoms and Rotifera, whereas the greatest part of 
plankton in the lakes and ponds is formed of crustaceans. It is noteworthy that 
in spite of the difference in relative abundance of the various forms, the organisms 
fo rmin g the plankton of the streams are the same as found in the plankton of stag- 
nant water. Therefore the term potamoplankton does not express the idea that 
there exists a special community of organisms adapted to live in the running water. 
Among the microscopical organisms there have been found only some species of 
Schizomycetes ( Micrococcus rhenanus, Sarcina alba, and Microspira danubica), 
which apparently occur exclusively in the streams. All other plankton forms in 
the river can be found also in the lakes, ponds, and pools. According to Steuer 
(1910) the potamoplankton can be characterized as an ecological group (Biocoenose) 
of organisms living and breeding in running water. This group consists principally 
of diatoms (Melosira, Asterionella, Synedra, Fragilaria, Stephanodiscus) and 
Rotifera (Asplanchna, Brachionus, Anuraea, Gastropus, Polyarthra, and Synchseta) . 
As one can notice from this list of organisms, which are regarded by Steuer as 
typical for river plankton, each of them can be found in stagnant water, and, as 
every limnologist knows, they all are very common in lakes and ponds. Their 
predominance in the river, however, may be taken as characteristic of the potamo- 
plankton, because such a combination of organisms has been observed in almost all 
rivers. 
It is obvious that the list of microorganisms that occur in the river plankton is 
not limited to the above-mentioned forms. Several observations have shown that 
some rivers carry exclusively zooplankton, but not phytoplankton, as is generally 
admitted. Thus Sernow (1901) observed that the Shoshma River, a tributary of 
