NET PLANKTON OF LAKE MENDOTA 53 



August, the highest percentages of this substance were found in spring 

 and in autumn, the latter always exceeding the former, and the lowest 

 percentages were noted in summer and in winter. 



The difference between the mean percentage of ash and the mean per- 

 centage of silica for the various years is shown in the last column of 

 table 14. This difference represents the other inorganic constituents 

 of the ash ; it was smallest in 1912 and largest in 1917. The maximum 

 range of variation was from 3.7 per cent in one sample of 1914 to 27.0 

 per cent in one sample of 1915. The general range, however, was much 

 smaller; in only four samples out of 184 did the difference between 

 the percentage of ash and the percentage of silica fall below 5.0 per cent 

 and in only 23 did it go above 15.0 per cent. Of the latter, 12 were 

 found in 1916 and 7 in 1917. 



Further analyses were made on the ash of 27 samples of net plankton 

 and the results of these analyses are shown in table 15, p. 190. 



Iron and Alumina (Fe^O^ and AI2O3). Quantitative determinations 

 of the iron and alumina were made on the ash of 16 samples. An at- 

 tempt was made to obtain quantitative results for them separately but 

 their amounts were so small in the material available for the work 

 that the analyses did not yield concordant results ; hence this effort was 

 discontinued and they have been recorded together in the table. The 

 iron and alumina constituted a minimum of 0.26 per cent of the dry 

 weight of the sample in one instance and a maximum of 2.18 per cent 

 in another sample. In 10 samples the amount did not exceed one per 

 cent while the mean for the 16 analyses is a little less than one per cent. 



Manganese (MugO^). The manganese was determined in the ash of 

 12 samples for the purpose of ascertaining how extensively the plankton 

 organisms make use of this element. Bradley* found that the tissues 

 of fresh-water mollusks belonging to the genera Anodonta and Unio 

 contain, on an average, slightly more than one per cent of manganese. 

 Since some of the plankton organisms included in these net catches 

 serve as food for these bivalves a few analyses were made in order to 

 determine the relative importance of this material as a source of this 

 element. No measurable amount was found in 5 of the samples; in 5 

 others the amount of Mn304 ranged from 0.30 to 0.39 per cent of the 

 dry weight of the material, while in the other two it was larger, namely, 

 0.75 and 0.77 per cent. Some of the net plankton, therefore, may serve 

 as a source of manganese for other organisms, but the amount available 

 in this material is relatively small. 



Phosphorus (P2O5). The phosphorus was determined in 21 samples 

 of ash. It was present in all of them and varied in amount from a 

 minimum of 0.25 per cent of the dry weight of the material to a maxi- 



*Jour. Biol. Chem., Vol. 3, pp. 151-157; Vol. 8, 1910-11, pp. 237-249. 



