376 
BULLETIN OF THE BUREAU OF FISHERIES. 
In discussing the plankton attention should first be called to the fact that the 
material suspended in water, which we find in our plankton net or on the filters, 
consists not only of organisms but also of organic detritus and mineral particles. 
These different constituents of the plankton sample can not be separated one 
from another, and therefore, in speaking of the volume of plankton, we must keep 
in mind that a part of this volume is formed by the mineral particles or by the 
products of decomposition of organisms, the so-called detritus. 
The different terms, such as euplankton, pseudoplankton, and detritus, have 
long been a subject of controversy. According to Hensen’s (1887) original defi- 
nition the word plankton denotes all that is floating in the water: “ Alles was in 
Wasser treibt.” The plankton-net catch contains, however, not only live organ- 
isms suspended in the water but also a certain quantity of inorganic matter such 
as sand and clay, as well as the products of decomposition of water plants and 
animals. In large and deep lakes the amount of inorganic matter and detritus 
is insignificant, but in shallow ponds and especially in rivers a great part of 
“plankton” samples consists of sand, silt, and detritus. Therefore when the 
productiveness of a basin is studied on the basis of volumetric determinations 
the examination of the detritus ought not to be omitted. 
The terms detritus and pseudoplankton have been used sometimes with the 
same meaning, while in other cases the term pseudoplankton has been applied to 
the bottom organisms that occasionally occurred in the pelagic region of a lake 
or sea. The term detritus also has been used with different meanings. In some 
papers it denotes the material of organic origin suspended in water, while in others 
it means all material suspended in water, with the exception of live organisms only. 
In 1917 Wilhelmi tried to set in order this terminology. He suggested the use of 
the term plankton strictly in accord with Kolkwitz’s definition of plankton as the 
natural community of those organisms that are normally living in water and are 
passively carried along by currents. Kolkwitz’s (1912) exact definition of plankton 
is as follows: “Die naturliche Gemeinschaft derjenigen Organismen welche in 
freiem Wasser, bei Stromung willenlos treibend, freilebend normale Existenzbe- 
dingungen haben.” As to the bottom and shore organisms that are only inci- 
dentally found in the pelagic region, these, according to Wilhelmi (1917), form 
pseudoplankton. All other material suspended in water, such as silt, sand, and 
particles of decomposed organisms, is called detritus, which, with regard to its 
origin, may be called inorganic or organic detritus. For this group of suspended 
substances Wilhelmi (1917) suggested a new term, “tripton.” The creation of a 
new term does not seem to help, and the old term “detritus” is as good as the new 
one, but of course the terms “ plankton ” and “ pseudoplankton ” must be applied to 
denote only the organisms and the term “ detritus ” must be used when speaking of 
all other material suspended in water. 
The water of the Mississippi River carries a- great amount of loose brown 
detritus which forms a great part of the material collected by filtering the river 
water through bolting silk. The amount of detritus depends chiefly on the stage 
of the river, increasing with the rise of the river and decreasing with the fall. 
Although the present observations have not been made simultaneously, still we 
are able to compare the results obtained in different parts of the river because 
