NANNOPLANKTON OF LAKE MENBOTA 79 



Pentosans 



The pentosans were determined in 32 nannoplankton samples which 

 were collected in 1915. The amount in them varied from a minimum 

 of 2.84 per cent of the organic matter to a maximum of 7.35 per cent ; 

 the mean percentage for these samples was 4.94 per cent. (Table 21, p. 

 193.) This represented a distinctly larger amount than was found 

 in the net plankton of this year, the mean percentage of the latter 

 being 3.41 per cent. With a larger amount of organic matter and a 

 higher percentage of pentosans in the nannoplankton than in the net 

 plankton the differences in favor of the former are still more striking 

 when the amounts are expressed in terms of milligrams per cubic meter 

 of water. In 1915, for example, the average quantity of the pentosans 

 in the nannoplankton was a little more than six times as much as that 

 in the net plankton. The maximum quantity in the former was about 

 three and a half times as large as the maximum of the net plankton, 

 while the minimum of the nannoplankton was more than fourteen times 

 as large as the minimum of the net plankton. 



Only eight determinations were made on the samples collected in 1916 

 and but two on those of 1917. The samples of 1916 yielded a higher 

 mean percentage of pentosans than those of 1915, but the two of 1917 

 were both lower than the mean of 1915. When stated in milligrams per 

 cubic meter of water the mean quantity in 1916 was about 10.0 per 

 cent higher and that of 1917 about 8.0 per cent lower than the mean 

 of 1915. 



Crude Fiber 



The percentage of crude fiber in the organic matter of the nanno- 

 plankton showed considerable variation during the period of these ob- 

 servations. The largest percentage was found in 1917 and the smallest 

 in 1915. (See table 22, p. 193.) The mean percentage for 1917 was 

 a little larger than that of 1916, while that of 1915 was less than half 

 as much as that of 1916. Crude fiber determinations were made on only 

 17 of the 41 samples obtained in 1916 and on only 4 of the 11 samples 

 of 1917, and this fact must be taken into account with reference to the 

 mean percentages. The differences between the mean quantities are of 

 similar magnitude when the results are expressed in terms of milligrams 

 per cubic meter of water. 



The mean percentage of the crude fiber in the nannoplankton in 1916 

 and 1917 was substantially the same as that of the net plankton for 

 1913 to 1915 inclusive, but it was much higher in the latter in 1911 and 

 1912. (See table 13, p. 189 for net plankton.) In 1915 the mean per- 

 centage of crude fiber in the net plankton was a little more than twice 

 as large as that of the nannoplankton ; in this year also the mean quan- 



