Zooplankton — Illinois River 



Table 2 shows that the number of zooplankters per liter in the 

 main channel of the Illinois River apparently declined by a factor of 

 8 between 1898 and 1974. The number of rotifers had declined dramati- 

 cally, while the copepods had increased slightly. Comparison of 

 Kofoid's 1898 results with those of WAPORA (1974: E8) in 1973, show 

 the same trends : 



Number Per Liter 



1898 c 

 386 



1973 L 

 36 



388 



Rotifera 

 Crustacea 



Branchiopoda 



Cladocera 

 Copepoda 

 Unidentified 



Total 



^ofoid, 1904: 314-340. 



b WAP0RA, 1974: E8. 



Present in very small numbers. 



The reasons for the decline of rotifers are not known, but one can 

 speculate that the decline in phytoplankton and the increase in the 

 suspended solids load of the lower Illinois River may have impaired the 

 feeding of rotifers. Rotifers are an important food for the fry of 

 many gamefish, such as bluegill, so the reductions noted above might 

 have had a significant impact on the growth and survival of fish fry. 



