54 PLANKTON OF WISCONSIN LAKES 



mum of 2.54 per cent, when calculated as P 2 5 . The average percentage 

 for the 21 determinations is 1.44 per cent. Schuette 5 reported the ele- 

 mentary phosphorus as varying from 0.91 to 1.57 per cent in 6 samples 

 of plankton which he analyzed. 



Sulphur. The quantity of sulphur was determined by Schuette in 

 8 samples of plankton; 5 of these samples consisted of a mixture of 

 plant and animal material, two were blue-green algae, and one was the 

 crustacean Daphnia pulex. One of the mixed samples contained only 

 0.42 per cent of sulphur, while the other 7 samples possessed substan- 

 tially the same percentage, namely, from 0.60 per cent to 0.64 per cent. 



Calcium (CaO). Calcium determinations were made on the ash of 

 23 samples. The amount varied from a minimum of 1.03 per cent to a 

 maximum of 6.8 per cent ; the average for the 23 samples was 3.2 per 

 cent. The plankton Crustacea, more particularly the Daphnias, show 

 a rather large variation in the amount of calcium which they possess 

 and this probably accounts for the marked differences in the net plank- 

 ton. 



Magnesium (MgO). The analyses of 24 samples show that the mag- 

 nesium is smaller in amount than the calcium. The magnesium in 

 these samples varied from a minimum of 0.17 per cent to a maximum of 

 1.95 per cent. The average for the series was 0.81 per cent, or only 

 about a quarter as large as the calcium. 



Results of Other Investigators 



Apstein 6 made ash determinations on the net plankton of several 

 fresh-water lakes. In the material from Dobersdorfersee he found a 

 marked variation in the percentage of ash during the different months 

 of the year ; it ranged from a minimum of 6.9 per cent of the dry ma- 

 terial in a catch which was obtained on August 31, 1891, to a maximum 

 of 45.0 per cent in a catch taken on October 4, 1891. In Ploner See a 

 minimum of 7.1 per cent was found on September 25, 1891, while on 

 November 6 a net catch yielded 66.6 per cent of ash. The lowest per- 

 centage of ash was found in net material collected in Molfsee on Au- 

 gust 18, 1895, namely, 2.9 per cent. 



The variations in the percentage of ash in the net plankton of Lake 

 Mendota were of about the same order of magnitude as those noted by 

 Apstein for Dobersdorfersee, that is, from a minimum of 5.6 per cent 

 to a maximum of 48.3 per cent. The net plankton of Molfsee yielded 

 a smaller percentage of ash than that of Lake Mendota and the material 

 from Ploner See gave a larger percentage. 



5 Trans. Wis. Acad. Sci., Arts, and Let., Vol. XIX, 1918, p. 602-3. 



6 Das Suesswasserplankton. Kiel und Leipzig, 1896, p. 200. 



