NANNOPLANKTON OF LAKE MENDOTA 71 



just two per cent. While the average amount of organic matter was 

 somewhat higher in the nannoplankton of 1916 than in that of the other 

 two years, it contained a smaller percentage of nitrogen. 



A comparison of the percentage of nitrogen in the net plankton 

 (table 7, p. 187) with that in the nannoplankton shows that the latter 

 is much lower in the dry sample, but when both are calculated to an 

 ash free basis the difference is relatively small, although the nitrogen 

 in the nannoplankton material is appreciably smaller than in the net 

 plankton. In 1915 when the observations covered all seasons of the 

 year, the mean percentage of nitrogen in the nannoplankton was less 

 than half of one per cent below that in the net plankton of 1911, but 

 it was almost one and a half per cent below that of the net plankton of 

 1915. The mean percentage of nitrogen in the nannoplankton in 1918 

 was almost two and a half per cent below that in the net plankton of 

 1915. 



The annual variations in the quantity of nitrogen in the nannoplank- 

 ton of Lake Mendota are shown in figure 29 ; the curves in this diagram 

 indicate the number of milligrams of nitrogen per cubic meter of water. 

 It will be noted that the general form of these curves is similar to that 

 of the curves representing the organic matter of the corresponding 

 years in figure 28 ; they show vernal and autumnal maxima with sum- 

 mer and winter minima. In 1915, however, there was an August maxi- 

 mum in both the organic matter and the nitrogen which exceeded the 

 autumnal maximum of that year. The curves for organic matter and 

 nitrogen are very similar in configuration in 1915, but the curves for 

 the other years show more or less marked differences. In 1916, for 

 example, the nitrogen did not rise to its maximum height in April 

 until a few days after the organic matter had passed its maximum 

 point; in the rise which took place during the latter half of June and 

 in early July the nitrogen reached its highest point during the last 

 week in June while the organic matter reached its highest point a week 

 later. The nitrogen curve has a well rounded form during November 

 and Dcember, 1916, while the curve for organic matter presents two 

 well defined peaks at this period. In 1917 the nitrogen curve possesses 

 two equally prominent peaks in the month of May, but in the curve for 

 organic matter the second peak is much lower than the first. 



In 1915 the ratio of the organic matter to the total nitrogen in the 

 nannoplankton of Lake Mendota varied from 9.1 to 18.8, thus showing 

 slightly more than a twofold difference. (See table 44.) In 1916 the 

 range was from 11.4 to 23.5, indicating a smaller proportion of nitrogen 

 in the organic matter obtained this year than in the previous year ; the 

 material collected in 1917 showed the smallest range of variation, 

 namely, from 9.4 to 16.9. For the entire series of nannoplankton sam- 



