go PLANKTON OF WISCONSIN LAKES 



tity was 49.0 milligrams per cubic meter of water in the latter and 22.0 

 milligrams in the former, somewhat more than a twofold difference. 

 The mean quantity of crude fiber in the samples of net plankton ob- 

 tained between 1911 and 1915 was only about a quarter to a sixth as 

 much as that of the nannoplankton material collected in 1916 and 

 1917. This difference was due mainly to the presence of a larger 

 quantity of nannoplankton than of net plankton. 



The crude fiber of 20 samples of nannoplankton were analyzed for 

 their nitrogen content, but it was found that the fiber contained at most 

 only a trace of nitrogen. This indicates that the organisms in this 

 material are practically free from chitin. 



Nitrogen Free Extract 



The nitrogen free extract, or that part of the organic matter which 

 is left after deducting the crude protein, the ether extract, the crude 

 fiber, and the ash, ranges from a minimum of a little less than 10.0 per 

 cent to a maximum of about 32.0 per cent of the dry weight of the 

 nannoplankton; that is, it shows a little more than a threefold varia- 

 tion as compared with sevenfold in the net plankton. The largest per- 

 centage of nitrogen free extract w^as found in a nannoplankton sample 

 collected in 1915 and the smallest percentage was noted in a sample ob- 

 tained in 1916. The mean percentage for the 87 samples of nanno- 

 plankton from Lake Mendota is approximately 19.0 per cent, or just 

 a little less than the mean for the net plankton. In the nannoplankton 

 the mean percentage of the pentosans is only a little more than one- 

 tenth as large as the mean of the nitrogen free extract and this is 

 substantially the same as noted for the net plankton. 



Ash 



The centrifuge material from Lake Mendota yielded a very high 

 percentage of ash, the range being from a minimum of about 35.0 per 

 cent to a maximum of about 75.0 per cent of the dry sample. In the 

 corresponding samples of net plankton the ash varied from about 9.0 

 per cent to a little more than 48.0 per cent of the dry material. Thus 

 the minimum of the nannoplankton was about four times as large as 

 the minimum of the net plankton, while the maximum of the former 

 was somewhat less than twice as large as that of the latter. 



The inorganic constituents of the nannoplankton were derived from 

 three sources, namely, (1) the nannoplankton organisms, (2) the silt 

 removed from the lake water by the centrifuge, and (3) the water re- 

 maining in the bowl of the centrifuge at the end of a run. In the latter 

 instance about 5.5 liters of lake water are involved ; that is, at the end 

 of each run this quantity of water remains in the bowl of the centri- 



