32 PLANKTON OF WISCONSIN LAKES 



In the general table, then, column one under nitrogen shows the 

 percentage of the total nitrogen, including that in the crude fiber ; but 

 the nitrogen of the crude fiber was deducted from the total nitrogen 

 before calculating the percentage on an ash free basis. In other words, 

 the results given in column two under nitrogen in the general table 

 represent the crude protein nitrogen in the organic matter. This same 

 correction applies to the next two columns, but the last column for 

 nitrogen shows the ratio of the organic matter to the total nitrogen. 



Table 7 (p. 187) shows the variations in the percentage of nitrogen in 

 the net plankton in the various years. Only about two-thirds of the 

 samples obtained in 1916 and 1917 contained enough material for a 

 nitrogen determination. In the total nitrogen there was approximately 

 a twofold variation in the percentage each year ; the ratio of maximum 

 to minimum is greater than two for the first three years and less than 

 two for the other four years. The greatest difference between maximum 

 and minimum was noted in 1913, namely, slightly more than five per 

 cent. The minimum percentage was found in August in each of the 

 complete years except in 1913 when it was noted in the month of De- 

 cember. In 1914 and 1917 the observations did not cover the latter half 

 of the year. In 1913 and 1914 the material which yielded the largest 

 percentage of nitrogen was obtained in the month of April; in 1915 and 

 1917 it was obtained in May ; in 1912 in June, in 1916 in July, and in 

 1911 it came in September. Thus in four of the five complete years 

 the maximum percentage of nitrogen was found either in the spring or 

 in the early summer. 



The second part of table 7 shows the variations in the crude protein 

 nitrogen when stated on an ash free basis. The ratio of maximum to 

 minimum each year is distinctly less than two ; the greatest difference, 

 almost five per cent, was noted in 1913, while the smallest differences 

 were found in 1915 and 1916, excluding 1917 which was not a complete 

 year. The maximum percentage of crude protein nitrogen was found 

 in 1913 and the minimum in 1916. The mean percentage was smallest 

 in 1911 and largest in 1917. Excluding the partial years 1914 and 

 1917, the largest mean percentage was found in 1915, but the smallest 

 mean, that of 1911, was a little less than one per cent below the maxi- 

 mum. Taken as a whole, then, the results for the five complete years 

 show a rather striking uniformity in the mean percentage of crude pro- 

 tein nitrogen in view of the composite character of the net plankton and 

 of the changes in the relative abundance of the various forms during 

 the different seasons of the year. 



The third column under nitrogen in table 43 shows the number of 

 milligrams of crude protein nitrogen per cubic meter of water. In the 

 various net samples on which nitrogen determinations were made, the 



