190 
BULLETIN OF THE BUREAU OF FISHERIES 
Thus, in spite of the wide range of the single ratios and the small number of 
observations, there is a close correspondence of average results in five different 
combinations. We are warranted in believing that the average nitrogen-carbon 
ratio is about 1 to 15. 
The organic carbon and nitrogen must be derived from (a) the plankton, (&) the 
higher plants growing from the bottom, (c) matter derived from the land and ulti- 
mately from the higher plants of dry land or marsh or from the fauna and flora of the 
soil. Thus the sources are similar, whether in water or on land. The proportion 
from each source must differ in different lakes and in the same lake at different times; 
but on the whole the plankton must be much the largest contributor. 
Birge and Juday’s plankton report (p. 215) furnishes data from which may be 
computed the nitrogen-carbon ratio in plankton algae, higher water plants, and 
plankton. The figures used for the percentage of carbon in crude protein, fats, and 
carbohydrates are the same as those employed later in the present paper (p. 196). The 
ratio of nitrogen to carbon was determined directly in 1 1 samples of plankton from 
Lake Mendota and was found to average 1 to 6.3, a sufficiently close agreement with 
the ratios given in the table as the result of computation from a much larger number. 
Table 4. — Ratio of nitrogen to carbon in algse, water 'plants, and plankton 
N 
C 
N 
C 
1 
5.5 
Vallisneria (1 analysis') 
1 
13. 5 
1 
9.0 
Plankton, Mendota (87 analyses) 
1 
7. 1 
1 
24.0 
Plankton, Monona (21 analyses) 
1 
5. 6 
1 
19.2 
Plankton, Wanhesa tlfi analyses') 
1 
6. 5 
Myriophyllum (1 analysis) 
1 
11.8 
We have no knowledge of the composition of the extractives from soil or from 
marshes, which are always present in lakes in some quantity and which give a high 
color to some waters. Russell (1923, p. 172) remarks on “the slow rate of disap- 
pearance of complex nitrogenous compounds in soil as compared with the rate of 
oxidation of carbon.” In land plants the ratio of nitrogen to carbon may be 1 to 
40; in the soil it is rapidly raised to 1 to 10. But we do not know at what point in 
this process these substances get into the lake. 
We are not able at present to show how the nitrogen-carbon ratio is established 
in the lakes. It is plain that the nitrogen of the dissolved matter is much smaller, 
relatively to the carbon, than it is in the plankton. It is plain also that the average 
ratio — 1 to 15, in round numbers — is not far from the ratio of 1 to 17 that comes 
from the four analyses of higher plants, as given in Table 4. The three general 
there analyzed yield about 88 per cent of the crop of aquatic plants in Lake Mendota. 
(Rickett, 1922, p. 525.) 
One might be tempted to infer that the low ratio of nitrogen to carbon is due 
to extractives from these plants, but such a conclusion would be wrong. The average 
standing crop of organic carbon in Lake Mendota is nearly 6.0 grams per cubic 
meter of water; the volume of the lake is 478,370,000 cubic meters. The lake, there- 
fore, contains nearly 2,900 metric tons of organic carbon. Rickett’s study of the 
larger aquatic plants (1922, p. 527) assigns to the annual crop for the entire lake a 
