NET PLANKTON OF LAKE MENDOTA 



39 



crease. The secondary peak in the curve for organic matter appearing 

 the first of August, 1912, has a sharp apex while that of the protein 

 curve at this time is broad. The same difference between the two curves 

 also appears during the autumnal maximum. In the vernal peaks of 

 1915 (fig. 14) the protein reached its highest point about a week later 

 than the organic matter so that the former curve has a distinctly- 

 broader summit. With the exception of these few minor differences, 

 however, the curves for organic matter and crude protein show a close 

 correlation during the period of time covered by these observations. 



r-t-m-4-^. 



^4H--rH>4-P'i-f IT 



Fig. 14. — The amount of dry organic matter, of crude protein and of ether ex- 

 tract in the net plankton of Lake Mendota in 1915. Curve A represents 

 the organic matter, curve B the crude protein and curve C the ether extract. 

 The curves show the number of milligrams per cubic meter of water. 



Column five under nitrogen in the general table (No. 43, p. 202) 

 shows the ratio of the organic matter in the net plankton to the total 

 nitrogen; that is, it is a numerical expression of the relation of the 

 former to the latter. As hiight be expected from the results given for 

 the percentage of total nitrogen in column one, this ratio is subject to 

 a twofold variation and it is relatively low because the percentage of 

 nitrogen is fairly high. In 91 out of 134 samples the ratio falls between 

 9 and 12; that is, in these samples the nitrogen constitutes from one- 

 ninth to one-twelfth of the dry organic matter. In two samples the 

 ratio falls below 9 and in 41 samples above 12, the highest being 17.4 

 and the lowest 8.7. 



