138. Mean total density for the main channel stations was 

 2.5/1 during July compared to a total of 28.0/1 during Septem- 

 ber .... This corresponds to the same trend observed for 

 phytoplankton among main channel stations. The obvious over- 

 riding factor limiting zooplankton is flow rate. Zooplankton 

 collected during high flow were from populations carried 

 downstream from calm backwater areas, where reproduction could 

 occur, and were greatly diluted in numbers by the rapid current 

 created by surface runoff. The most abundant group found in 

 the highly turbid waters characteristic of the July sampling 

 period were the Copepoda, and they were present at a mean 

 density of only 1.5/1. 



139. The September sampling period followed a 49-day period 

 of average river stage at Alton, Illinois . . . and the main 

 channel was more lake-like due to closing of navigation dams. 

 The observed density increase during this period was related 



to an increase in density of all three phyla, particularly 

 Rotifera, which increased more than 20 times the density 

 observed for this group in July .... 



140. Mean number of taxa increased by a factor of four 

 between sampling periods .... This increase is attributed 

 to improved habitat conditions brought about by decreased flow 

 and turbidity and increased phytoplankton abundance. 



Zooplankton diversity and density also increased in side channel 

 habitats between July and September, due to a reduction in flow during 

 August and September (Colbert e_t al. , 1975: 65). 



Phytoplankton — Illinois River 



Between June 12, 1894 and March 28, 1899 Dr. Charles A. Kofoid 

 took 235 samples of plankton from the main channel of the Illinois 

 River near Havana, at mile 128.5 (Kofoid, 1903: 291-340; Kofoid, 1904: 

 314-340), and in 1899 made one survey which included the lower 80 

 miles of the Illinois (Kofoid, 1903: 273-283). Biologists from WAPORA 

 (WAPORA, 1974) took plankton samples from the Illinois River at 

 Meredosia in September, 1973. The only post-dam survey of the plankton 

 in the study reach of the Illinois River we have been able to locate 

 was conducted by Colbert et al. (1975) in 1974. Although the two surveys were 

 separated by a span of seventy-five years , the sampling methods used were strikingly 

 similar, and the results are probably comparable, for the reasons given below. 



We used Kofoid 's data for the year 1898 (Kofoid, 1904: 314-340) 

 for comparison with the 1974 data of Colbert et al . (1975: E29-E36) . 



11 



