343 



(Table IV.), with a maximum of .032 on August 25. The 

 amounts reported are all very small, and the significance of 

 their differences is questionable. The following correlations 

 with environmental conditions may be noted. There is little 

 plankton (.004) in the turbid (3 cm.) flood water of August 18 ; 

 there is more (.032) in the clearer water (30 cm.) of August 25. 

 The production following the rapid decline of temperature in 

 September falls to a minimum (.002) as it does in the channel 

 waters (.53 and .23), and like the latter rises again (.OOcS) late 

 in October, after a month of somewhat stable temperatures 

 (PI XXII.). The December production (.002 and .001), how- 

 ever, shows no rise corresponding to that in the main stream. 



The average production in Spoon River for the five months 

 in 1896 in which collections were made there is only .007 cm. 

 per m.*, while that in the main stream is 97 times as great, the 

 production there exceeding that in the tributary from 4-fold to 

 380-fold in each month (see table between pp. 342 and 343). 

 Spoon River water is thus throughout this season a diluent of 

 the channel plankton. 



The chemical conditions during this period reveal unutil- 

 ized nitrates averaging 1.2 parts per million in Spoon River to 

 1.15 in the Illinois. Other forms of nitrogen are somewhat 

 more abundant in the main stream. There is, however, plenty 

 of food for the plankton in the tributary, and other causes than 

 poverty of nutrition must be cited to explain its paucity of 

 plankton. 



1897. 



(Tables IV., XI. ; PI. XXVIIL, XLVI.) 



There are 13 collections in this year, at intervals of two 

 to six weeks. They average 1.257 cm.' per m.\ and have a 

 maximum of 7.296 on September 11. The conditions attending 

 the unusual plankton production in Illinois River channel 

 waters in this year affect Spoon River also in much the same 

 manner. The vernal overflow mingled impounded backwaters 



