NANNOPLANKTON OF LAKE MENDOTA 73 



pies from Lake Mendota, the ratios varied from 9.1 to 23.5 ; that is, the 

 nitrogen constituted from one-ninth to abont one-twenty-third of the 

 organic matter in the centrifuge material. This was a larger variation 

 than was found in the net plankton of Lake Mendota in which the 

 extremes were 8.7 and 17.4. 



In 1915 several determinations of the nitrogen content of the water, 

 both before and after being centrifuged, were made for the purpose of 

 ascertaining how closely the loss of nitrogen during the centrifuging 

 process corresponded with the amount of nitrogen in the nannoplankton 

 recovered by the centrifuge. It was found that the individual deter- 

 minations showed considerable variation ; in some instances the loss of 

 nitrogen by the water was in excess of that recovered in the nanno- 

 plankton and in other samples the reverse was true. These differences 

 were due, apparently, to slight inaccuracies in the method of determin- 

 ing the quantity of nitrogen in the water when it is present in such 

 relatively small amounts. These variations are about evenly balanced, 

 however, so that the differences become comparatively small in the aver- 

 ages for a number of determinations. 



In twelve analyses, for example, the average quantity of nitrogen 

 lost by the water in the centrifuging process was 106.3 milligrams per 

 cubic meter of water, and the nannoplankton material recovered from 

 this water yielded 103.5 milligrams per cubic meter; the two amounts 

 correspond as closely as could be expected when the two very different 

 methods of treatment are taken into consideration. That is, the dis- 

 crepancy is only 2.8 milligrams, or a difference of 2.6 per cent. A 

 larger number of determinations would, doubtless, have reduced this 

 difference materially. 



In these twelve analyses the average quantity of organic nitrogen in 

 the water after it was centrifuged was 758.0 milligrams per cubic meter ; 

 in comparison with this the nannoplankton yielded 103.5 milligrams 

 and the net plankton 21.5 milligrams, thus making a total of 125.0 milli- 

 grams of organic nitrogen per cubic meter of water for the total plank- 

 ton. On this basis the water of Lake Mendota contained somewhat 

 more than six times as much organic nitrogen as the total plankton. In 

 addition to this the water contained an average of 4.0 milligrams of 

 nitrite and 392.0 milligrams of nitrate so that the total nitrogen 

 amounted to 1,154.0 milligrams per cubic meter of water, all of which 

 would be available for the nitrogenous element in the food of the va- 

 rious plants of the lake. According to these results, then, the water 

 contained a little more than nine times as much nitrogen as the total 

 plankton. 



In view of the abundance of the dissolved nitrogen compounds, 

 Putter 's theory regarding the nutrition of aquatic animals may be con- 



