ORGANIC CONTENT OP LAKE WATER 
191 
dry weight of 2,100 metric tons, of which less than one-half would be organic carbon. 
Thus, the annual carbon crop of the higher plants of Lake Mendota is hardly more 
than one-third of the average standing crop of dissolved organic carbon. 
The situation in Green Lake is even more unfavorable to large contributions 
from the higher plants. The smaller figure in Table 2 for organic carbon in this lake 
is 3.95 grams per cubic meter; the volume of the lake is 973,000,000 cubic meters; 
and on this basis the lake would contain more than 3,800 metric tons of organic 
carbon. Rickett (1924) found in Green Lake an annual crop of higher plants, 
yielding about 1,528 metric tons of dry matter; but almost exactly half of this is in 
the form of Chara, which contains more than 50 per cent of ash. An outside estimate 
of the annual crop of organic carbon from the higher plants of this lake would be 
500 to 600 metric tons. 
It is equally clear that inflowing water brings only relatively small quantities 
of organic matter into the lakes, and the plankton necessarily is left as by far the 
chief source of the dissolved matter. Other facts of secondary importance look the 
same way. If extractives from the higher plants were influential in controlling the 
amount of organic carbon and nitrogen, then the spread between these substances 
ought to be greater in late fall and winter than in spring and early summer; but 
this is not the case. Again, the centrifuge plankton ought to show the influence 
of vegetable debris at the same seasons; but this also is not the case. The small and 
shallow lakes should show a larger proportion of carbon than the larger and deeper 
bodies. This also does not appear; on the contrary, in Lake Michigan, whose water 
must receive only minimal supplies of organic matter from affluents and from plants 
of shallow water, the carbon is twenty-two and seven-tenths times the nitrogen, 
being exceeded in this respect by only one lake. 
Much further study will be needed before the intricate story of the organic matter 
of lakes is finally unraveled. But at present we may conclude that the plankton is 
the chief source of the dissolved organic matter, and that its nitrogen compounds 
disappear more rapidly than its nonnitrogenous substances, thus reversing the 
situation as stated by Russell for the land. This conclusion might be expected in 
view of the rapid accumulation of carbohydrates in peat under water, as compared 
with the very slow growth of humus under subaerial conditions. 
ETHER EXTRACT 
The ether extract from several of these samples was determined by Dr. B. P. 
Domogalla in 1925 and 1926. Six samples of single residues were treated; of these, 
four were from Mendota — two surface and two bottom water — one sample was from 
Green Lake, and one from Lake Geneva. Ether extracts also were made from 2 
large composite samples from Lake Mendota; 10 residues taken from the surface 
water at various dates were combined into 1 sample, and 6 residues from the bottom 
water made up the other. In the composite sample from surface water 2 specimens 
were taken in February; 2 others, April and November, during the homothermous 
period; the other 6 were taken during the period of summer stratification — 2 in July, 
2 in early October, and 1 each in August and September. The 6 bottom samples 
