288 



distance from the east and west shores respectively. As will be 

 seen in Plate XL, this was made after nine weeks of uninter- 

 rupted low water, when the river had been standing at 2 ft. 

 for some time. The catches were made between 2 and 4 

 o'clock p. m. There was no vegetation in the river at this 

 point in this season, though both Havana Lake and Quiver 

 Chute, to the north (PI. II.), contained a small amount. The 

 discharge from Quiver Creek and Lake makes its way along 

 the eastern margin of the river, while that of Spoon River un- 

 der these hydrographic conditions hugs the western shore. The 

 effect is seen in the turbidity records, the clearer water be- 

 ing on the eastern side and the more turbid on the western. 



The following table gives the data of collection. There 

 was almost no silt in the catch, and the silt estimates are 

 therefore omitted. 



PLANKTON IN CROSS-SECTION OF ILLINOIS RIVER BELOW MOUTH OF SPOON RIVER. 



Num- 



Temperature 





Tur- 

 bid^' 

 depth 



{in m.; 



Plankton per 



Plankton under 



Departure from mean 

 in per cent. 



ber of 



(F.) 



Depth 

 in m. 



ni.3, in cm. 3 



1 sq. meter 



(omitting- Nos. 1 and 2) 



lec- 



Sur- 



Bot- 



Vol- 



Departure 



Vol- 



Departure 



Per m. 3 



Under 1 



tion. 



face 



tom 







ume 



from mean 



ume 



from mean 



sq. m. 















in per cent. 





in per cent. 







1 



71.5 



70 



.66 



.35 



2.40 



-60 



1.58 



-83 







2 



71.5 



70 



1.06 



.33 



3.88 



-35 



4.11 



-57 







3 



70.5 



70 



1.42 



.35 



5.40 



-10 



7.67 



-20 



-20 



-32 



4 



70.5 



70 



1.58 



.30 



6.32 



+ 5 



9.99 



+ 4 



- 6 



-12 



5 



70.3 



70 



1.58 



.31 



5.60 



- 7 



8.85 



- 8 



-17 



-22 



6 



70.5 



70 



1.68 



.25 



7.64 





r27 



12.84 



+34 



+14 





-14 



7 



70S 



70 



1.83 



.25 



8.20 





L37 



15 00 



+56 



+22 





-33 



8 



70.5 



70 



1.88 



.22 



7.40 





1-23 



13.91 



+45 



+10 





-23 



9 



71 



70.5 



1.83 



.20 



6.84 





-14 



12.52 



+30 



+ 2 





-11 



10 



71.2 



71 



l.r2 



.15 



6.28 





- 5 



9.55 



- 1 



- 6 



-15 





Average 







I 6.00 



±22.3 



| 9.60 I ±33.8 



! ±12.1 ! ±20.2 



The results of this test are confirmatory of the thesis here 

 maintained, namely, that the distribution of the plankton in a 

 stream does not differ in the main from that thus far observed 

 in lakes in the matter of variations in the plankton content 

 (volumetric) in different localities. The average ± depar- 

 tures from the mean volume, computed per m. 3 and under 1 

 sq. m., in these ten catches are 22.3 and 33.8 respectively, as 

 compared with 31.8 and 28.8 for Lake St. Clair and 5.52 for 

 the German lakes examined by Apstein. 



