Table 2 



Comparison of Zooplankton Populations In the Main Channel of 



the Illinois River in 1898 and 1974, Under Stable Low-flow Conditions 



in September 3 



Major Taxonomic Divisions 

 Rotifera (rotifers) 

 Crustacea (crustaceans) 



Number of 

 Species 



1898 b 1974 c 



29 



10 



Number of 



Individuals 



Per Liter 



1898 c 



336 



1974* 



25 



^)ata were obtained from Kofoid, 1904: 314-340, and Colbert et al. , 1975: 

 F16-F20. Although Kofoid 's samples were taken upstream of the study 

 area, he conducted four studies (Kofoid, 1903: 273-283) of the longitudinal 

 distribution of plankton in the river (one study included the lower 80 

 miles of the Illinois) and concluded that the plankton in the main chan- 

 nel was remarkably uniform. Hence, Kofoid 's samples from the Havana area 

 are probably generally representative of the main channel in the fall of 

 1898. Both Kofoid and Colbert et al. (1975) used no. 20 plankton nets. 

 Kofoid also collected plankton on filter paper, but these results are 

 excluded from the table because Colbert et al. did not use micro-filtration. 



The total number of species taken on Sept. 6, 13, 20, and 27, 1898 at 

 one sampling station in the Illinois River near Havana (mile 128.5). 



The total number of species taken at six sampling stations (miles 23.1, 

 45.6, 57.6, 58.3, 77.0, and 81.0) in the lower Illinois River. The 

 station at mile 2.5 was excluded because the number of species was much 

 lower than at the other six stations. 



The total number of individuals taken in the four collections in 

 September, 1898, was divided by four to obtain an average density. 



The total number of individuals taken at six locations (excluding the 

 atypical results from river mile 2.5) was divided by six to obtain an 

 average density. 



Present in very small numbers. 



19 



