186 
BULLETIN OF THE BUREAU OF FISHERIES 
I. DATA 
The data of this paper came from five sources: 
1. An elaborate study of the quantity and chemical composition of the plankton 
of Lake Mendota and the other Madison lakes. This was made by Birge and Juday 
(1922) from 1911 to 1917. The results of this work are used in this paper to furnish 
data regarding the average amount of plankton from Lake Mendota and also to give 
its composition. This paper is cited as the “plankton report.” 
2. A study of the quantity and the forms of nitrogen found in lake waters, 
made primarily by Dr. B. P. Domogalla (Domogalla, Juday, and Peterson, 1925). 
This study was based on the same samples as those employed in the present paper; 
it is the source of the nitrogen determinations of plankton and residues given in 
the present paper. It is cited as the “nitrogen report.” 
3. A more complete study of the forms of nitrogen found in a small number of 
these waters, made primarily by Prof. W. H. Peterson (Peterson, Fred, and Domo- 
galla, 1925). This is used only in some of the final conclusions of the present paper. 
4. A determination of the organic carbon contained in some of the residues 
originally prepared for the work of Doctor Domogalla. These results are published 
for the first time in the present paper. The determinations were made by Dr. C. R. 
Wise, of the department of chemistry, University of Wisconsin. 
5. Determinations of the ether extract made by Doctor Domogalla on a number 
of the same samples. These also are reported here for the first time. 
DISSOLVED ORGANIC NITROGEN AND ORGANIC CARBON 
The figures for organic nitrogen in this section of the paper are taken from the 
nitrogen report (Domogalla, Juday, and Peterson, 1925), for which all of the samples 
originally were secured. 
The methods of collecting and handling the samples of lake water are reported 
fully in this paper (p. 270) ; they contained from 50 liters to 525 liters of water. The 
plankton was extracted by a Sharpies centrifuge running at a speed of 40,000 revo- 
lutions per minute, which removed the plankton, including about 70 per cent of the 
bacteria. The centrifuged water was evaporated in a copper vacuum pan at a tem- 
perature of about 60° C. The plankton nitrogen was determined from the dried 
material extracted by the centrifuge, and six forms of nitrogen were regularly deter- 
mined from the dry residues resulting from evaporation. For the inorganic nitrogen 
these were free ammonia, nitrate, and nitrite; and for the organic nitrogen there were 
determined the free-amino nitrogen, peptide, and nonamino nitrogen. 
All of this nitrogen from the residues is called “dissolved,” and the same term 
is applied to the organic carbon from the same source. In the use of this word the 
present paper follows the terminology of the nitrogen report, which considers nitrogen 
under two categories — plankton nitrogen and soluble nitrogen. It is not intended 
that inference should be made from the use of either word as to the exact condition 
in which organic substances, other than plankton, exist in the water. No doubt 
they are present in various forms, including (a) that part of the plankton and the 
organic debris that escapes the centrifuge, (6) colloids of different kinds, and (c) true 
solutions. These necessarily are treated together as they are mingled in the dry resi- 
