NANNOPLANKTON OF LAKE MENDOTA 65 



cance in the final results because they constitute a relatively small part 

 of the bulk of the centrifuged material. 



Three sets of experiments were made in 1915 to determine what effect 

 the centrifuging process had on the chemical composition of the ma- 

 terial. The results of these tests are shown in. table 16, p. 191. Net 

 plankton, consisting largely of algae, was used for the experiments. A 

 fairly large catch of plankton was obtained by means of a net and the 

 material was placed in about two liters of water. The catch was then 

 stirred thoroughly in order to distribute the organisms as evenly as 

 possible and it was divided into two equal parts. One portion was 

 evaporated to dryness directly and used as a standard sample. The 

 other portion was put into about 900 liters of centrifuged water and the 

 material was then recovered with the centrifuge; its subsequent treat- 

 ment was the same as that given to the regular centrifuge catches. 



In samples No. 5108 and No. 5109 Coelosphaerium was the predomi- 

 nant form with Ceratium second ; a relatively small portion of the ma- 

 terial consisted of Microcystis and several other algae. In samples No. 

 5120 and 5121 Coelosphaerium was also the most abundant form but 

 there was a very large element of Lyngbya and Microcystis. The bulk 

 of the material in samples No. 5154 and No. 5155 consisted of diatoms, 

 chiefly Melosira and Stephanodiscus. In these catches the blue-green 

 algae were represented mainly by Microcystis with an occasional fila- 

 ment of Lyngbya. These catches also contained a larger proportion of 

 zooplankton than the other two sets ; Chydorus and Anuraea cochlearis 

 were the leading representatives of this group. 



In total organic matter the uncentrifuged material in sample No. 

 5109 exceeded that in the centrifuged half of the catch by 311.6 milli- 

 grams. This difference in favor of the uncentrifuged portion was due 

 to two factors. With such a large amount of material, namely, about 

 22 grams dry weight, concentrated into a little less than two liters of 

 water, it was difficult to distribute the organisms evenly so that the 

 catch could be divided into two equal portions ; experiments have shown 

 also that there is danger of a slight loss of material from the centrifuge 

 whenever the catch exceeds about 10 grams in dry weight. In spite of 

 these unfavorable factors, however, it will be noted that the difference 

 is less than three per cent. In samples No. 5120 and No. 5121 there 

 was a small difference, less than two per cent, in favor of the centri- 

 fuged portion of the catch. Sample No. 5154, the centrifuged portion, 

 yielded just 34.0 milligrams less than No. 5155, a difference of less than 

 one-half of one per cent in favor of the uncentrifuged material. 



In table 16 the nitrogen and ether extract are indicated in percent- 

 ages of the dry organic matter in the various samples. The centrifuged 

 and uncentrifuged portions of each set of experiments show substan- 



