128 PLANKTON OF WISCONSIN LAKES 



Ash. The ash constituted from 6.68 per cent to 30.73 per cent of the 

 material in the various samples, with a mean of 10.86 per cent. The 

 percentage was highest in spring and in autumn when the diatoms were 

 most abundant and lowest in the summer when the green and blue- 

 green algae predominated. (Table 47.) 



In most of the catches silica was the most important constituent of 

 the ash, varying in amount from 1.04 per cent to 23.52 per cent of the 

 dry material. The silica was notably high in those samples containing 

 a large percentage of ash, thus indicating the presence of diatoms in 

 such samples. j -k ^ul W } j ' If H 



Deducting the percentage of silica from the percentage of ash leaves 

 from 4.85 per cent to 14.87 per cent of the dry sample for the other 

 inorganic constituents of the ash. The samples obtained in 1913 and in 

 1915 yielded a smaller amount of such constituents than those collected 

 in 1916. The first sample taken in 1916, No. 633, gave the maximum 

 difference, 14.87 per cent ; the two samples following this one showed a 

 slight decline in the percentage of the other inorganic constituents. 

 The most marked decrease was noted between samples No. 667 and No. 

 679, taken on June 29 and July 11, in which the difference fell from 

 12.15 per cent in the former to 7.91 per cent in the latter. Thereafter 

 the decline continued until the difference between the percentage of ash 

 and the percentage of silica amounted to only 5.21 per cent on August 

 23, or but little more than a third as much as the maximum of May 24. 

 In September and October of this year the difference ranged from 7.21 

 per cent to 9.60 per cent. 



The ash of one sample, No. 352, was subjected to a further analysis 

 and the following results are stated in terms of percentages of the dry 

 plankton material: ash 22.78 per cent, silica 16.37, Fe 2 3 and A1 2 3 

 1.35, P 2 5 0.67, CaO 2.20, and MgO 1.28 per cent. This sample was 

 obtained on September 19, 1913, when the diatoms were fairly abundant 

 and the silica derived from their shells constituted more than two-thirds 

 of the ash, leaving only 6.41 per cent for the other inorganic consti- 

 tuents. 



Organisms of the Net Plankton 



The net plankton of Lake Waubesa was made up of Crustacea, a very 

 few rotifers, some protozoa, and various forms of algae. The Crustacea 

 were represented by species of Diaptomus, Cyclops, Daphnia, and 

 Chydorus. The rotifers included a few individuals belonging to the 

 genera Asplanchna and Triarthra, and two species of Anuraea. The 

 flagellate Ceratium was noted in all of the material. 



The algae were represented by a rather large variety of forms. 

 Among the blue-greens the most important forms were species of Ana- 



