8UMMABY AND DISCUSSION 157 



weight of the different kinds of organisms per individual or colony, is 

 of prime importance in the solution of this complex problem. Data 

 regarding the first two questions will enable one to estimate, with some 

 degree of accuracy, the annual turnover in the stock of plankton ma- 

 terial, and therefore the annual production of this material, while data 

 pertaining to the third question will make it possible to evaluate the 

 relative importance of the different forms in the plankton complex of 

 the lake. 



^ While these data do not indicate the rate of turnover in the stock of 

 plankton, yet some computations based wholly on estimates may be 

 worthy of consideration at this point. The mean quantity of dry or- 

 ganic matter in the standing crop of total plankton of Lake Mendota 

 amounted to approximately 945 metric tons during the period covered 

 by these observations, or an average of 240 kilograms per hectare of 

 surface (214 pounds per acre), when the entire lake is taken into ac- 

 count. A turnover in this mean quantity of plankton organic matter 

 once a month would give an annual production of 2,880 kilograms per 

 hectare, or 2,568 pounds per acre; a turnover twice a month would 

 double this amount. 



Since the planktonts which contribute by far the greater part of the 

 organic matter multiply rapidly under favorable conditions, once a day 

 or oftener, the turnover in this stock of plankton material may take 

 place more frequently, perhaps as often as once a week, on an average. 

 With a turnover of fifty times per year, the annual production would 

 amount to 12,000 kilograms of dry organic matter per hectare of sur- 

 face, or 10,700 pounds per acre. Which of the above amounts ap- 

 proaches the annual production most closely can not be determined with 

 any degree of accuracy until more data are available relating to the 

 rate of reproduction of the various plankton organisms in their natu- 

 ral environment. It seems most probable, however, that the time inter- 

 val for the turnover in the mean quantity of plankton will be found 

 to fall somewhere between one and two weeks during the greater part 

 of the year. 



