110 PLANKTON OF WISCONSIN LAKES 



results are summarized in the second part of table 33. Silica is present 

 in largest amounts when diatoms are most abundant, usually in spring 

 and in autumn. A high percentage of silica was found on August 15, 

 1916, which was due to a large crop of diatoms at that time. 



The silica varied from a minimum of 0.12 per cent of the dry weight 

 of the sample in one catch obtained in 1913 to a maximum of 27.05 per 

 cent in one of the catches of 1916. The mean percentage was lowest 

 in 1911 and highest in 1916, but there was no regular increase between 

 these dates as already indicated for the ash. In general the mean 

 percentage of silica was much lower in the net plankton of Lake Monona 

 than in that for the corresponding years from Lake Mendota. 



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

 centage of silica of the same year represents the average of the other 

 inorganic constituents of the ash. This difference is shown in the last 

 column of table 33. The smallest dift'erence is indicated for 1912 and 

 the largest for 1916, the latter being more than twice as large as the 

 former. There is the same marked increase in this difference in the 

 later years over the earlier ones corresponding more or less closely to 

 the increase in ash. In 1913 and in 1916 the differences were substan- 

 tially the same for Lake Monona as for Lake Mendota, but in the other 

 years they were lower in the former lake. 



The ash of two samples of net plankton from Lake Monona was sub- 

 jected to a further analysis and the results are shown in table 15, p. 190. 

 Both of these samples were collected in 1913, one on July 30 (No. 330) 

 and the other on September 24 (No. 354). The ash amounted to 6.20 

 per cent of the dry sample in the former catch and to 12.16 per cent in 

 the latter. The marked difference was due mainly to a difference in the 

 amount of silica in the two samples. The latter sample also contained 

 a distinctly larger proportion of Fe203 and AL-Og than the former. De- 

 ducting the percentage of silica and the percentage of iron and alumina 

 from the percentage of ash in each sample reduces the difference be- 

 tween the two to only 0.71 per cent. Of the remaining constituents, 

 P2O5 and CaO are the most important. It will be noted, also, that the 

 percentage of MgO in sample No. 330 is four times as large as that of 

 No. 354. The variations in the percentages of the different constituents 

 of these two ashes, however, come within the range of those noted in 

 the ash analyses of the net plankton of Lake Mendota. 



Organisms of the Net Plankton 



The Crustacea were well represented in the net plankton of Lake 

 Monona. The copepods included species of Diaptomus and Cyclops to- 

 gether with their nauplii ; there were five forms of Cladocera, namely, 

 Daphnia pxdex, D. hyalina, D. retrocurva, Chydorus, and Leptodora. 



