NET PLANKTON OF LAKE MENDOTA 45 



percentages of crude protein, ether extract, crude fiber and ash are 

 deducted from 100 and the remainder constitutes the percentage of 

 nitrogen free extract. 



There is a wide variation in the percentage of this extract in the 

 net plankton of Lake Mendota, ranging from a minimum of about 5.0 

 per cent to a maximum of 35.0 per cent ; in the great majority of the 

 samples, however, it falls between 10.0 per cent and 25.0 per cent. The 

 average for the entire series of net samples is a little more than 20.0 

 per cent of the dry weight of the material. 



Analyses of substantially pure catches show that the nitrogen free 

 extract in ten samples of blue-green algae constitutes from 25.0 per cent 

 to 52.0 per cent of the dry weight of the material ; in most of the sam- 

 ples it falls between 30.0 per cent and 40.0 per cent. Two samples of 

 diatoms show approximately 23.0 per cent and 34.0 per cent, respec- 

 tively. 



Table 49 shows that the crude protein, ether extract, crude fiber, and 

 ash constitute from 75.0 per cent to 95.0 per cent of the dry weight of 

 the plankton Crustacea ; the former was noted for a sample containing 

 Daphnia pulex and the latter in one consisting of Cyclops. These four 

 items, therefore, account for a much greater proportion of the crusta- 

 cean material than they do of the algal material. 



A quantitative study of only one of the carbohydrate compounds of 

 the net plankton was made, namely, of the pentosans. The analyses 

 show that the pentosans constitute but a relatively small part of the 

 carbohydrate material that is present in the samples. The average for 

 all of the pentosan determinations, for example, is about 2.6 per cent, 

 while that of the nitrogen free extract is 20.8 per cent, the former 

 being only about one-tenth as large as the latter. 



Ash 



The ash content of the net plankton was relatively large, more espe- 

 cially when diatoms were abundant. In only 12 samples out of a total 

 of 184 did the ash fall below 10.0 per cent of the dry weight and in only 

 79, or less than half of the total number, did it fall below 20.0 per cent. 

 In 68 out of 100 samples obtained from Lake Mendota between 1911 and 

 1914 the ash fell below 20.0 per cent, while only 11 out of 84 secured 

 between 1915 and 1917 fell below this percentage. Table 14 gives a 

 summary of the ash and silica determinations of the different years, 

 while all of the determinations are indicated in the general table (No. 

 43, p. 202). The differences between maximum and minimum percen- 

 tages of ash in the various years range from less than twofold to almost 

 sevenfold. The former was noted in 1914, in which year the observa- 

 tions were discontinued on July 1, so that a full season was not repre- 



