PLANKTON OF LAKE MONONA 109 



Nitrogen Free Extract. The crude protein, ether extract, crude 

 fiber, and ash were all determined on 27 of the 41 samples of net 

 plankton from Lake Monona so that the nitrogen free extract can be 

 computed for this number of catches. Together these four items con- 

 stituted a minimum of only about 51.0 per cent of the dry material in 

 sample No. 345, thus leaving 49.0 per cent for the nitrogen free extract 

 consisting of the carbohydrates that were not included in the crude 

 fiber. 



The maximum percentage of crude protein, ether extract, crude 

 fiber, and ash was found in sample No. 5167 in which they constituted 

 almost 85.0 per cent of the dry weight of the sample, so that substan- 

 tially only 15.0 per cent consisted of nitrogen free extract. These 

 results show that the nitrogen free extract in the net plankton of Lake 

 Monona ranges from a minimum of about 15.0 per cent of the dry 

 material to a maximum of 49.0 per cent, or somewhat more than a 

 threefold variation. 



Ash. The lowest percentage of ash found in the 47 samples of net 

 plankton from Lake Monona amounted to 2.28 per cent of the dry 

 weight of the material in sample No. 345, August 27, 1913, and the 

 highest was 33.19 per cent in sample No. 6111, August 15, 1916 ; thus, 

 there was almost a fifteenfold difference between the maximum and the 

 minimum. Table 33 (p. 199) gives a summary of the ash determina- 

 tions and the complete data are given in the general table, No. 45. The 

 lowest percentages were found from late July to early September, 1913, 

 and the highest ones in the year 1916. Of the 13 samples obtained in 

 1916 the percentage of ash amounted to 20.0 per cent or more in 8 of 

 them, while in the other 34 samples of the series only 5 contained 20.0 

 per cent of ash or more. In general, the net plankton of Lake Monona 

 contained a distinctly smaller percentage of ash than that of Lake 

 Mendota. ^-wm^ 



The net plankton of Lake Monona showed a considerable variation in 

 the maximum and minimum percentages of ash from year to year, but 

 the general result was a marked increase in the mean percentage of ash 

 from 1911 to 1916 ; each year showed an appreciable increase over the 

 previous one. The mean for 1916, for example, is more than two and 

 a half times as much as that of 1911, the difference amounting to 13.51 

 per cent. A similar rise in the mean percentage of ash in the net 

 plankton of Lake Mendota was noted between 1911 and 1915, with the 

 exception of 1914; the mean for 1915 was 11.44 per cent higher than 

 that of 1911. (See table 14, p. 190). 



Silica. The percentage of silica was determined for all but two of 

 the net catches from Lake Monona ; in these two instances the material 

 was accidentally lost before the determination was completed. The 



