525 



COMPARISON OF MONTHLY MEANS OF PLANKTON PRODUCTION IN ILLINOIS 

 AND SPOON RIVERS, BASED ON ALL CATCHES.* 



*Amounts in this table are cubic centimeters of plankton per cubic meter of 

 water after subtracting from the total catch the estimated amount of silt. 



meters. This is 4.7 times as much as the average of all the 

 Spoon River collections — 11.5 times, if we omit the two 

 exceptional collections of September 11 and 30, 1897, and 44 

 times, if we exclude the low-water period of the autumn of 

 1897 from both averages. This latter ratio, 1 to 44, represents 

 the relative plankton content of the two streams except during 

 periods of prolonged and extreme low^ water. Even in such 

 low-water periods the waters of the tributary are less pro- 

 ductive than those of the main stream. For example, the aver- 

 ages of the monthly averages of production for July-October, 

 inclusive, in the two streams are respectively 2.89 and 5.78 cm.' 



