196 
BULLETIN OF THE BUREAU OF FISHERIES 
Thus the facts presented in Table 2 complete the proof, if such were needed 
that the fresh-water lake offers an unrivaled opportunity for the study of plankton 
in its relations to the fundamental food supply of water animals. The lake contains 
not only a complex association of living organisms, each group existing in numerous 
representatives and forms and in quantities great enough for study, but it contains 
also a great amount of organic material not removable by centrifuge and apparently 
not existing in particulate form. In this matter of quantity the fresh water has a 
great advantage over the sea. Studies on the water of the Mediterranean (Henze, 
1908), the Baltic (Raben, 1910), and the Irish Sea (Moore et ah, 1912) agree in 
finding that the total amount of organic carbon in the water of the open sea is so small 
that it lies “at the limits of detectability by the best known methods” (Moore 
et al., p. 269). The difficulty of studying this minute amount is increased further 
by the great quantity of salts dissolved in the water. 
The water of lakes, on the other hand, contains organic matter, extractable by 
the centrifuge, in quantities ordinarily present to the amount of 1 milligram per 
liter; often as great as 5 to 7 milligrams per liter; and sometimes much greater. It 
contains, also, organic matter not thus extractable, whose quantity is ordinarily five 
to ten times as great. The dissolved salts of fresh water are small in amount, and 
there are many lakes whose dissolved organic content is larger than that of the 
dissolved salts. Thus, the fresh water far exceeds the salt in its opportunities for 
the study of the numerous and complex problems offered by its organic content. 
As yet the science of limnology hardly has begun to deal with these problems. 
For years there has been an active discussion of the question whether the organized 
plankton is distributed to the two classes of eaters and eaten so as to make them 
into a self-sustaining community. But as yet there is no agreement on the quantity 
of organic matter contained in either group, on the rate of its renewal, or on the 
amount, kinds, and availability of organic matter in the water but outside of organized 
beings. 
TOTAL ORGANIC CONTENT 
In this section the organic content of lake water is discussed without taking 
into account the ether extract. The amount of this and the effect on the statement 
of total content are discussed in the next section. 
The data for the plankton content of the water are given in Table 6, which 
gives total content and nitrogen. The “crude protein,” the nitrogenous part of the 
plankton, may be computed from the nitrogen by employing the standard factor — 
6.25. The nonnitrogenous part is the remainder of the organic matter after the 
crude protein has been subtracted. The samples of plankton were too small to 
permit the determination of the ether extract, and therefore the nonnitrogenous 
substance can not be subdivided further into fats and carbohydrates. The plankton 
of the Madison lakes has been so analyzed in the tables of the plankton report. 
In the study of the dissolved organic matter it is not possible to separate this 
from the relatively large quantities of inorganic salts that make up the bulk of the 
residues from evaporated lake water. Recourse must be had to indirect methods, 
based on the data of Table 2, which states the nitrogen and the organic carbon. 
The nitrogenous part of the dissolved matter — the crude protein — is computed 
as in the case of the plankton by multiplying the amount of nitrogen by 6.25. The 
