490 



MoNTHivY Means of Gage Readings 

 Illinois River, Havana 



We have thus two reasons for making the year 1897-98 the basis 

 of our comparison of plankton production before and after the open- 

 ing of the drainage canal ; and as that for the former year is 3 

 centimeters per cubic meter and for the latter is 5.07 centimeters, 

 we have sufficient reason for concluding that the plankton produc- 

 tion of the stream has been largely increased per unit of volume as 

 a consequence of the opening of the drainage canal, and that, so far 

 as the evidence goes, the ratio of the present yield to the former is 

 at least as 5 to 3. 



Remembering further that the most important plankton product 

 is that of the spring months, when young fishes are hatching from 

 the egg and are dependent upon the plankton organisms for their 

 earliest growth as fry, we find a special significance in the fact that 

 the plankton yield of the river for the months of March, April, May, 

 and June, was 2.85 times as great in 1910 as it was in 1898; or, if 

 we omit the month of June as rather late for this purpose, the yield 

 of the former year, March to May, is still 2.12 times that of the 

 latter year. Taking for our comparison all the plankton averages for 

 March, April, and May in all the years from 1894 to 1899, we find 

 that the yield of these months in 1910, averages 4.29 times that of 

 the same months of the earlier period. 



From these various points of view, therefore, we are compelled 

 to infer a much larger plankton yield to the cubic meter of water 

 in the Illinois River itself at Havana during the year September, 



