ORGANIC CONTENT OF LAKE WATER 
203 
in the water three times as much as exists in the plankton, perhaps four times as 
much as the plankton offers as available food for Crustacea and rotifers. These 
soluble proteins are as nutritious, so far as their chemistry gives evidence, as are 
those of the plankton. The question of their use, therefore, is a question of fact 
to be settled by study, just as the question of the utilization of algse as food by 
Daphnia must be settled. 
The same statements may be made regarding the nonnitrogenous dissolved 
matters, with a confidence proportional to our knowledge or ignorance of the re- 
lation of these substances to the corresponding elements of the plankton. Outside 
of the centrifugible plankton there is four times as much ether extract as within it; 
the organic carbon assignable to carbohydrates is ten times as great outside of the 
plankton as within it. 
It is evident that many questions are raised here, both of fact and theory, to 
which there are no answers at present. The story of the organisms and the organic 
substances of the lake water is comparable in its difficulties and complexities to the 
story of the cells and the organic matters in the human blood. It will take time 
to work out the story of the lake, and it may be long before we know the nature, 
the origin, the renewal, and the fate of the substances that we have here called 
dissolved. 
It may be even harder to read the story of the possible or actual utilization of 
these substances as food. We may fairly assume that they have the potentialities 
of food; but dissolved matter in proportions of 6 to 30 in a million parts of water 
makes a very dilute nutriment, unless for creatures that have some special apparatus 
for selecting just these materials out of the water. 
The carbon dioxide of the atmosphere, from which the land plants derive then- 
carbon, constitutes 1 part in 2,500 by volume and more than this by weight; and of 
this, the carbon is more than one-fourth. In the water of Lake Mendota the organic 
matter of the dissolved protein constitutes only 1 part by weight in about 400,000 of 
water; the average total organic carbon, instead of being more than 1 part in 10,000, 
is less than 1 part in 150,000; and this great dilution is in a fluid far less mobile than 
air and one in which there is only a small relative movement between organisms 
and surrounding medium. 
SUMMARY 
1. The water of Lake Mendota contains in its plankton an average of about 
0.140 milligram per liter of nitrogen and about 0.990 milligram of organic carbon. 1 
1 A very large series of determinations of the amount of organic carbon and nitrogen in fresh water is to be found in the sixth 
report of the English co mm ission on pollution of rivers, presented to Parliament in 1874 (House of Commons Documents, 1874, 
vol. 33). This report contains many hundreds of determinations of organic carbon and nitrogen from potable waters, made by 
Prankland and Armstrong. Twenty-three lake waters were examined, chiefly from soft-water lakes. The mean amount of organic 
carbon found in these waters is 2.30 milligrams per liter; the organic nitrogen is 0.314 milligram per liter; the nitrogen-carbon ratio 
is N ; C :: 1 : 7.2. If the organic matter is computed on the same basis as in the present paper the result is nitrogenous, 1.96 milli- 
grams per liter; nonnitrogenous, 2.80 milligrams per liter; total organic matter, 4.76 milligrams per liter. 
This report has received little notice from limnologists. The methods of analysis were slow and complicated and did not come 
into general use. In some cases they might lead to inaccurate results. Our attention was called to the report by a reference in 
a paper by W. H. Pearsall, entitled “Phytoplankton and environment in the English lake district” (Revue Algologique, March, 
1924). He evidently accepts the data of the report for the lakes to which he refers; and, in general, the report shows conditions quite 
similar to those found by us in soft-water lakes of northeastern Wisconsin during the field seasons of 1925 and 1926. Pearsall’s paper 
was received after the present paper had gone to press 
