346 



(PI. XLVL, Table XI.) show somewhat uniform conditions in 

 strong contrast with the instability in these particulars in the 

 Illinois. With greater stability Spoon River apparently pro- 

 duces a more abundant plankton in this low-water autumn. 



The exceptional production in this autumn stands in 

 strong contrast with the poverty of this tributary in the same 

 months in 1896 and 1898. As seen in the table between pp. 

 342 and 343, the maximum monthly production in the low 

 water of 1897, as recorded in monthly averages, is from 285 to 

 to 5,130 times greater than that in any month in this season in 

 the other two years. A comparison of the data in Table XI. 

 for the three years in question and of their plottings on Plates 

 XLVL and XLVII. will show the great similarity in the chem- 

 ical conditions which accompanies this remarkable inequality 

 in production. The accompanying table summarizes the data 

 concerning nitrogenous contents of the water and the plank- 

 ton. 



NITROGENOUS SUBSTANCES AND PLANKTON, 1896-1898. 

 MONTHLY AVERAGES— PARTS PER MILLION. 



Year 



1896 

 1897 



Other factors than these chemical conditions are thus re- 

 sponsible for the great differences in production in these three 

 years. Low water, slight or imperceptible current, and conse- 

 quent time for breeding in the latter part of 1897 are the most 

 probable factors in causing the high production of that sea- 

 son, while in other years the recent origin of the tributary wa- 

 ter, from rains or springs, precludes any considerable produc- 

 tion in water otherwise capable of supporting an abundant 

 plankton, as is shown by the production in 1897. 



As a whole. Spoon River plankton in 1897 reflects the same 



