150 PLANKTON OF WISCONSIN LAKES 



dry organic matter per unit of area, namely, 665 kilograms per hectare 

 (594 pounds per acre), while Waubesa is second in rank with 503 kilo- 

 grams per hectare (448 pounds per acre) and Mendota is third with 

 435 kilograms per hectare (388 pounds per acre). When the results 

 are computed on the basis of the volume and area of the entire lake, 

 Monona is again first in rank with 267 kilograms per hectare (238 

 pounds per acre), Waubesa is second with 241 kilograms per hectare 

 (215 pounds per acre), and Mendota is third with 240 kilograms per 

 hectare (214 pounds per acre). 



The mean percentage of nitrogen is largest in the total plankton of 

 Lake Monona and smallest in that of Lake Mendota. The percentage of 

 ether extract, on the other hand, is largest in the material from Lake 

 Mendota and smallest in that obtained from Lake Waubesa ; the latter 

 lake has the highest mean percentage of pentosans and Lake Mendota 

 the largest mean percentage of crude fiber. 



Discussion of Results 



For the sake of emphasis it may be worth while to repeat the state- 

 ment that the data pertaining to the quantity of material per unit of 

 volume or of area refer to the dry weight of the organic matter and that 

 the living organic matter would weigh about ten times as much. The 

 total weight of the living material would also include the inorganic 

 constituents of the ash. Just how much the ash of the nannoplankton 

 organisms would add to the weight of the living organic material in 

 these organisms can not be determined from the present data because 

 the centrifuge catches contained a certain amount of inorganic material 

 derived from the silt in addition to the ash of the organisms. Since the 

 quantity of inorganic material yielded by the nannoplankton organisms 

 is not known, the amount of ash in the total plankton can not be in- 

 dicated. 



A number of determinations show that the ash content of the plank- 

 ton algae ranges from about 3.0 per cent to almost 10.0 per cent of the 

 dry weight in the green and blue-green forms, and from 40.0 per cent 

 to more than 50.0 per cent in the diatoms. In Euglena and Volvox 

 the ash constitutes from 4.0 per cent to 9.0 per cent of the dry weight. 

 It seems probable, therefore, that the ash of most of the nannoplankton 

 organisms does not constitute more than 10.0 per cent of the dry 

 weight, but the diatoms yield a much larger percentage. When diatoms 

 are abundant in the nannoplankton in the spring and in the autumn, 

 the percentage of ash will be relatively high but when the diatoms are 

 at a minimum in summer and in winter it will be comparatively low. 



The mean quantity of ash in the 184 samples of net plankton from 

 Lake Mendota amounts to 23.5 per cent of the dry weight of the ma- 



