346 
(Pl. 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 seenin the table between pp. 
342 and 348, the maximum monthly production in the low 
water of 1897, as recorded in monthly averages, is from 285 to 
to 5,180 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 
XLVI. 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. 
November December 
Bg | 2S | meh ee Re Ba ceil ee | eee 
= S w + = = eG Po) om + 
veer Se | Ge] e Wee dgehee | ea) eee 
BE Oe | el ce ee ee |e 
TOQO sce resco .065 | .84 | .013 | 1.6] .005 | .o1 -52 | .008 | 1.3 | .002 
PSO] Jearcani seater 052 | .76 | .008 56/1 .671 022| .68 | .005 | .63 | .599 
1898 jects neni 026 | .37_ | .009 -58] .oor | .084| .89 | .o15 | .33 | .oor 
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 
