NANNOPLANKTON OF LAKE MENDOTA 81 



fuge and large numbers of organisms are suspended in it. During 1915 

 all but about 250 cubic centimeters of this water was siphoned out of 

 the bowl and kept separate from the material which was deposited on 

 the side of the bowl. The amount of organic matter represented by 

 the organisms suspended in the bowl water was estimated by making a 

 quantitative determination of the organic nitrogen in this water. The 

 quantity of nitrogen was then multiplied by the factor representing the 

 ratio of the organic matter to the nitrogen in the sample. By this method 

 of treatment only about a quarter of a liter of lake water was added to 

 the sample, distilled water being used to wash the catch from the bowl ; 

 this small quantity of lake water did not add materially to the ash of 

 the sample. 



In 1916 and 1917, on the other hand, all of the bowl water was added 

 to the sample and the entire quantity was evaporated ; this made a very 

 substantial addition to the ash. Several determinations showed that 

 an average of 119.0 milligrams of inorganic material was obtained from 

 a liter of lake water, so that the 5.5 liters contributed about 655.0 milli- 

 grams of ash to the sample. Ash determinations on aliquot portions of 

 18 bowl waters in 1915 gave an average of 652.0 milligrams of inorganic 

 material for 5.5 liters, thus corresponding closely with the former 

 amount which was ascertained by another method. On the basis of 

 these results it has been assumed that the bowl water contributes 655.0 

 milligrams of ash to the sample, or twice this amount where two runs 

 are combined into one sample. 



The percentage of ash in the various organisms of the nannoplank- 

 ton, with the exception of Euglena, has not been determined, but such 

 results have been obtained for various constituents of the net plankton 

 and these may be used as a basis for estimating the ash of the nanno- 

 plankton. In some of the blue-green algae the ash varied from 4.3 

 per cent to 7.8 per cent of the dry weight, while in Euglena it amounted 

 to 5.1 per cent. In the diatoms, of course, there is a much larger pro- 

 portion of ash, ranging from 40.0 per cent to approximately 50.0 per 

 cent. Since the diatoms constitute a relatively small portion of the 

 centrifuge material, except in the spring and in the autumn, the ash 

 derived from the organisms in the summer and winter catches may be 

 estimated as about 10.0 per cent ; that is, the organic matter constitutes 

 90.0 per cent of the dry material in these organisms. On this basis, 

 then, the ash derived from the nannoplankton organisms ranged from a 

 minimum of about 100.0 milligrams to a maximum of approximately 

 250.0 milligrams per cubic meter of water during June, July, and Au- 

 gust, while in January, February, and March the amount varied from 

 90.0 milligrams to about 158.0 milligrams per cubic meter. In the 

 spring and in the autumn small diatoms and the fragments of the larger 



