ORGANIC CONTENT OP LAKE WATER 
201 
was taken at one level; but while the amount of dissolved matter varies, it shows 
no such differences with depth as does the plankton. 
NATURE OF THE DISSOLVED ORGANIC MATTER 
No special study has been made of the ether extract from the residues, but its 
appearance under the microscope shows that probably it is composed of normal fats. 
The exact nature and the nutritive value of the carbohydrate material have not 
been studied, either in the plankton or the dissolved matter. Something more 
definite may be said regarding the crude proteins. 
The nitrogenous compounds of the plankton of Lake Mendota have been com- 
pared with those from the dissolved matter. Two composite samples of plankton 
were made up, one from the net plankton and the other from the nannoplankton. 
Each was made up from 10 catches, representing all seasons from April, 1915, to 
June, 1917. The organic nitrogen was determined by the methods used in the study 
of the residues, and it was separated into the three forms that were standard for the 
residues — free-amino nitrogen, peptide nitrogen, and nonamino nitrogen. The 
result is given in Table 12, where the analyses of the plankton are compared with the 
average results from the residues, as given by Domogalla, Juday, and Peterson (1925, 
pp. 273, 274). The residues include 29 samples of water from the surface and 21 
from the bottom. 
Table 12. — Percentile distribution of organic nitrogen from Lake Mendota 
Nitrogen 
Plankton 
Residues 
Net 
Nanno 
Total 
Surface 
Bottom 
Total 
Free amino 
Per cent 
20.5 
50.5 
29.0 
Per cent 
16.3 
39.5 
44.2 
Per cent 
17.0 
41.4 
41.6 
Per cent 
20.0 
39.8 
40.2 
Per cent 
20.4 
38.9 
40.7 
Per cent 
20.3 
39.3 
40.4 
Nonamino - 
Total.. 
100.0 
100.0 
100.0 
100.0 
100.0 
100.0 
The net plankton contains nearly all of the animal matter that comes from 
Crustacea and rotifers; in computing the total results it has been given a weight of 
17.4 per cent, corresponding to the average relation existing between net plankton 
and nannoplankton. 
It appears, therefore, that the main classes of nitrogenous compounds exist in 
the residues in almost exactly the same proportions as in the plankton; there is 
much less difference between the means, as given in the table, than exists between 
the observations that go to make up the several means. In 15 samples from other 
lakes (nitrogen report, p. 282) the forms of soluble nitrogen showed much the same 
kind of variation as in Lake Mendota; the average of the 15 cases was as follows: 
Free-amino nitrogen, 17.2 per cent; peptide nitrogen, 38.7 per cent; nonamino 
nitrogen, 44.1 per cent. 
The forms of nitrogen in the residues have been analyzed still further by Peter- 
son, Fred, and Domogalla (1925, p. 293). It appears that such indispensable amino 
acids as tryptophane, cystine, tyrosine, histidine, and arginine are present in the 
