NET PLANKTON OF LAKE MENDOTA . 31 



during these observations. It exceeded the next in rank, namely, the 

 autumnal maximum of 1912 by 80.0 milligrams per cubic meter of 

 water. 



The autumnal period is succeeded by a decline to a winter minimum 

 which is reached in late January, as in 1914, or by the middle of Febru- 

 ary, as in 1917. In 1913, however, the autumnal maximum was fol- 

 lowed by a secondary rise which reached its highest point during the 

 third week in December; this was then followed by a decline to the 

 winter minimum. After reaching the winter minimum the quantity of 

 organic matter remains fairly constant until about the middle of April 

 and soon after this date the vernal increase begins. 



CHEMICAL RESULTS 

 Nitrogen and Crude Protein 



The detailed results for the nitrogen determinations on the net plank- 

 ton of Lake Mendota are shown in table 43 (p. 202). There are five 

 columns in the general table pertaining to the nitrogen. The first one 

 shows the percentage of the total nitrogen in the dry sample ; the second 

 gives the percentage of nitrogen in the organic matter exclusive of that 

 found in the crude fiber ; the third indicates the number of milligrams 

 of nitrogen per cubic meter of water; the fourth shows the quantity 

 of crude protein, that is, the milligrams of nitrogen multiplied by the 

 protein factor 6.25 ; the fifth column indicates the ratio of the organic 

 matter to the nitrogen. The curves marked B in figures 11 to 14 show 

 the number of milligrams of crude protein per cubic meter of water in 

 the net plankton. 



A certain portion of the total nitrogen is found in the chitin of the 

 shells of the plankton Crustacea ; since these shells pass through the ali- 

 mentary tract of fishes without being affected, apparently, by the di- 

 gestive processes, it is assumed that this nitrogen compound has no food 

 value. It was necessary, therefore, to make a correction for the nitro- 

 gen in the chitin since one of the purposes of this investigation was to 

 secure data regarding the food value of the plankton. Schuette 1 found 

 that the crude fiber derived from the plankton Crustacea yielded from 

 5.9 per cent to 6.2 per cent of nitrogen, or substantially the same per- 

 centage as reported for chitin by several investigators. So the correc- 

 tion was made by determining the percentage of nitrogen in the crude 

 fiber of the plankton material and then deducting this amount from the 

 total nitrogen. It has been reported that chitin exists in the walls of 

 many blue-green algae also, but later investigations do not confirm the 

 earlier results ; thus its presence in these forms is still an open question. 



1 Trans. Wis. Acad. Sci., Arts, and Let., Vol. XIX, 1918, p. 610. 



