345 
(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 em.) 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 (.008) late 
in October, after a month of somewhat stable temperatures 
(Pl. 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 em. 
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 mainstream. 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.; Pl. XXVIII., 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 
