PLANKTON OF LAKES WAUBESA AND KEGONSA 139 



Total Organic Matter per Unit Area. The plankton material was ob- 

 tained in the deepest portion of Lake Waubesa, that is, in the region 

 where the water reaches a maximum depth of a little more than 11 

 meters. The 10 meter contour line bounds an area of 101.2 hectares and 

 the volume of water within this boundary line is 10,306,000 cubic 

 meters. In this region of the lake, then, the maximum crop of total 

 plankton in 1915, namely, 9,489.7 milligrams per cubic meter of water, 

 amounted to 966 kilograms of dry organic matter per hectare, or about 

 862 pounds per acre. The living organic matter would represent about 

 9,660 kilograms per hectare, or 8,620 pounds per acre. 



The mean quantity of organic matter for the 16 samples of total 

 plankton is 4,938.3 milligrams per cubic meter of water, which repre- 

 sents 499 kilograms of dry organic matter per hectare of surface, or 

 445 pounds per acre. 



For the entire lake the maximum crop of total plankton in 1915 

 represented 465 kilograms of dry organic matter per hectare, or 415 

 pounds per acre, and the mean of the 16 samples amounted to 242 

 kilograms per hectare, or 216 pounds per acre. The quantity per unit 

 area for the entire lake was somewhat less than half as large as that 

 for the deep water. 



LAKE KEGONSA 



The material from Lake Kegonsa consisted of a single sample of net 

 plankton which was obtained on July 29, 1913. This catch gave the 

 largest yield of net plankton per cubic meter of water that was secured 

 during the entire investigation (see table 2, p. 181) ; it was almost 

 one-third larger than its nearest competitor. Owing to the high per- 

 centage of ash in this material, namely, 38.85 per cent of the dry sample, 

 with the silica amounting to 32.38 per cent of the dry material, the 

 quantity of organic matter per cubic meter fell below that of a net 

 sample taken in Lake Waubesa on September 10, 1915. (See table 47, 

 p. 213, sample No. 5129 and table 25, p. 196.) In this net catch from 

 Lake Kegonsa the phytoplankton was much more abundant than the 

 zooplankton and the chief constituent of the former was Melosira. The 

 ash content of this diatom was high, as indicated by the high percentage 

 of silica, and this served to reduce the proportion of organic material 

 in the sample. 



The percentage of nitrogen in the organic matter of the net catch 

 from Lake Kegonsa was somewhat lower than the average for the net • 

 material from the other lakes, namely, 6.88 per cent as compared with 

 7.54 per cent for Lake Waubesa and 9.36 per cent for Lake Monona. 

 (Table 25.) Thus the crude protein constituted a proportionally smaller 

 percentage of the organic matter. The large quantity of organic matter 

 per cubic meter of water, on the other hand, gives a larger yield of ni- 



