NET PLANKTON OF LAKE MENDOTA 35 



to bring out more clearly the vernal and autumnal maxima of nitrogen 

 as well as the summer and winter minima. Many of the net catches 

 that were made in 1916 and 1917 did not contain enough material for 

 a nitrogen determination so that the data for these years are not com- 

 plete enough for the construction of curves covering this period. Thus, 

 only one winter season is covered in these curves, namely that of 1913-14 

 (fig. 10), but it furnishes a good illustration of the winter minimum 

 which extended from late January, 1914, to the latter part of March. 



The various curves reach their greatest heights in the spring in the 

 months of May and June, while the summer minima are found in July 

 and August. The autumnal maximum in the four years covered by 

 these observations was attained in the month of October. These curves 

 show clearly that the quantity of crude protein nitrogen was smallest in 

 1911, less than 10.0 milligrams per cubic meter of water being found 

 from the middle of June to the middle of September. The quantity 

 of nitrogen remained well above 10.0 milligrams per cubic meter of 

 water during the summer of 1915 ; it fell below this amount only once 

 in the summer of 1912 and twice in the summer of 1913. 



Column four under nitrogen in table 43 (p. 202) shows the quantity 

 of crude protein per cubic meter of water ; that is, it gives the results 

 obtained by multiplying the quantity of nitrogen shown in column 

 three by the protein factor 6.25. The various proteins do not all pos- 

 sess the same percentage of nitrogen, the amount ranging from a mini- 

 mum of about 15.0 per cent to a maximum of 19.0 per cent, but this 

 factor is the one generally used by food chemists for calculating their 

 nitrogen determinations into terms of protein. Thus, the results given 

 in this column are more or less conventional, but they serve to give a 

 general idea of the quantity of crude protein. 



The range of variation in the quantity of crude protein in the various 

 years is shown in the second part of table 8 (p. 188) in which the 

 maximum, minimum, and mean amounts are stated in milligrams per 

 cubic meter of water ; the ratio of the maximum to the minimum is also 

 indicated in the last column of this table. 



The smallest maximum was noted in 1914, but this was due, no doubt 

 to the fact that the observations were discontinued on the first of July 

 so that the autumn maximum was not obtained. The largest maximum 

 was found in 1912, while that of 1911 was somewhat less than half as 

 large. The maxima of the other years fell between these two. The 

 smallest minimum was noted in 1911 and the largest in 1915. Likewise, 

 the smallest mean was found in 1911 ; it was only about 45.0 per cent as 

 large as that for 1913 which was the next smallest of the four complete 

 years. The mean for 1915 was the largest, the amount for this year 

 being almost two and a half times as large as that for 1911. 



