547 



contributions of tributary streams are continuously sown with 

 organisms whose further development produces in the Illinois 

 River a plankton as yet unsurpassed in abundance. 



TOTAL ANNUAL PRODUCTION. 



In the absence of any precise figures of the total annual 

 discharge of the Illinois River at its mouth or at Havana, it is 

 futile to attempt to compute with any considerable degree of 

 accuracy the total plankton production of this stream or of any 

 of its tributaries or backwaters examined by us. The estimates 

 which follow, contain, therefore, a large element of conjec- 

 ture. 



On page 132, computations from available data indicate 

 a run-off at Havana of approximately 0.688 second-foot per 

 square mile of the drainage basin of 15,250 square miles. 

 The mean of all the monthly averages (see table p. 429) of 

 plankton production in channel waters is 2.71 cm.^ per m.^ of 

 water. On this basis the total discharge in the average year 

 becomes 25,408.3 cubic meters of plankton permanently re- 

 moved each year by the discharge of the stream at Havana. 

 From the stream as a whole, given the same plankton content 

 and run-off per square mile as at Havana, the total production 

 becomes 67,598.8 cubic meters of plankton. These figures rep- 

 resent in a measure the unutilized and permanently lost organic 

 matter available for the support of the larger, more permanent 

 animals resident in the river and its connecting waters. They 

 are the net production over and above that plankton utilized by 

 the organisms which retain their residence in the river or its 

 backwaters above Havana or the mouth, and those whose death 

 and decay have removed them from the plankton by sedimen- 

 tation or solution. The total production is therefore greater 

 in some unknown and apparently undeterminable ratio. 



It is evident that production varies greatly from year to 

 with the varying plankton content and discharge of the stream. 

 In the accompanying table I have computed the total annual 

 production of the Illinois River on the basis of a run-off of .688 



