192 
BULLETIN OF THE BUREAU OF FISHERIES 
included in the composite came 1 in February, 2 in July, 2 in September, and 1 in 
early October; all but the first were in the period of summer stratification of water. 
Table 5. — Ether extract, milligrams per liter 
Lake 
Depth, 
meters 
Date 
Dry 
matter 
Ether ex- 
tract 
Single sam - 
Mp.ndnta . _ ...... 
0 
167.8 
0 323 
Do 
0 
Sept. 18, 1924 _ 
169 0 
380 
Do 
20 
Sept. 26, 1922 
162 8 
488 
Do 
20 
172. 1 
688 
Green 
0 
July 18,1923 _ 
143. 7 
362 
Geneva ... 
0 
July 16| 1923 
114. 0 
. 217 
Composite 
samples 
Mendota.. . 
0 
10 samples 
164. 3 
0 482 
Do 
20 
149.7 
.765 
The range of ether extract, as shown from these specimens, is from about 0.2 
milligram per liter to nearly 0.8 milligram. No doubt a larger series would show a 
greater range. The quantities obtained were not large enough to permit further 
chemical study, but under the microscope they were seen to be made up of clear 
droplets of oil. 
This ether extract must have its main source in the plankton, just as the nitrog- 
enous substances of the dissolved matter come from similar substances in the 
plankton. The ether extract has been determined for many samples of the plankton 
from the Madison lakes (plankton report, pp. 196, 202-209). In Lake Mendota 
the average amount is 0.149 milligram per liter, or 7.53 per cent of the ash-free 
organic matter. This may be compared with the average result of the 20 samples 
of residues from Lake Mendota, which yield 0.564 milligram of fat per liter, or 
4.4 per cent of the total organic matter as computed. 
The range of ether extract in plankton is great. It extends from 0.004 milli- 
gram per liter of lake water to 0.136 milligram in the case of the net plankton, and 
from 0.018 milligram to 0.356 milligram in the case of the centrifuge plankton. 
Thus, the fats of the dissolved organic matter found in a unit of volume of water 
are much greater than those in the plankton from the same amount of water. How- 
ever, they constitute a smaller per cent of the dissolved matter than of the plankton. 
The nitrogen and carbon were determined for the two composite samples from 
Lake Mendota. The results, stated in milligrams per liter, are as follows: 
Surface 
Bottom 
0.457 
5.580 
0.452 
6. 350 
1 : 13. 1 
1 : 14. 0 
