. PLANKTON OF LAKES WAUBESA AND KEGONSA 135 



representing almost a twofold variation. The percentage was some- 

 what larger in the nannoplankton material from Lake Waubesa than in 

 that from Lake Monona where the minimum was 14.0 per cent and the 

 maximum 23.0 per cent. 



On an ash free basis the nitrogen free extract constituted 42.8 per 

 cent of the dry organic matter in sample No. 666 and 50.7 per cent in 

 sample No. 597. From 30.7 per cent to 52.7 per cent of the organic 

 matter in the twelve samples from Lake Waubesa consisted of nitrogen 

 free extract. The minimum was found in samples No. 5128 and No. 

 5174 and the maximum in sample No. 690. 



Ash. The ash in the centrifuge catches from Lake Waubesa ranged 

 from a minimum of 33.44 per cent to a maximum of 65.35 per cent of 

 the dry material. (Table 48.) The average for the entire series 

 amounted to 49.38 per cent, or substantially the same as that noted for 

 the centrifuge samples from Lake Monona. 



The diatoms were fairly abundant in all of the nannoplankton sam- 

 ples from Lake Waubesa except four and these four were probably the 

 only ones in which the ash of the organisms fell within the 10.0 per 

 cent limit ; two of these samples were obtained in June, 1916, No. 650 

 and No. 666, and two in July, 1916, No. 678 and No. 690. Allowing 

 10.0 per cent for the ash, the organisms in these four samples contrib- 

 uted from 89.0 milligrams to 253.0 milligrams per cubic meter of 

 water to the total ash of the samples. When the ash derived from the 

 organisms and that derived from the bowl water are deducted from the 

 total ash, the remainder, which may be attributed to silt, varies from 

 368.0 milligrams in sample No. 650 to 1,135.0 milligrams per cubic 

 meter of water in sample No. 678 ; this means from 0.37 to 1.13 parts 

 of silt per million parts of water. These results show that silt is a little 

 more abundant in the water of Lake Waubesa than in that of Lake 

 Monona, but it is substantially the same as that in Lake Mendota. 



Organisms op the Nannoplankton 



The protozoan population in the nannoplankton of Lake Waubesa 

 consisted of essentially tKe same forms that were found in Lake Mo- 

 nona. There was a somewhat greater variety of algal forms, however, 

 than in Lakes Mendota and Monona. The more abundant algae were 

 Aphanocapsa delicatissima, Oocystis, and the young colonies and frag- 

 ments of Microcystis and Anabaena ; several other forms were obtained 

 from time to time in the various catches, but they were never found 

 in any considerable numbers. The chief diatoms were Melosira and 

 Stephanodiscus. 



Chlorochromonas was abundant in the four catches which were ob- 

 tained in 1915; it ranged from a minimum of 135,000 per liter of 



