Williams (1964: 813, 815) reported peak, numbers of diatoms in the 

 Illinois River at Grafton were 1,600,000 to 3,199,000 per liter in 1961- 

 62, and at Peoria were 6,400,000 to 12,799,000 per liter in 1960-61 

 and 3,200,000 to 6,399,000 per liter in 1961-62. Williams' (1964: 811) 

 collecting techniques captured all cells 4 microns or larger. His 

 numbers are considerably higher than those obtained by other investigators, 

 who used collecting techniques which would retain only larger diatoms 

 and other types of algae. For example, WAPORA (1974: 11) used 10-micron 

 mesh nanoplankton nets and Colbert e_t al . (1975: 25) used No. 20, 

 81-micron mesh plankton nets. Although Starrett (1972: 152) compared 

 Williams' 1960-62 data at Peoria to unpublished data collected in 1964, 

 1969, and 1970, it is not certain that the sampling methods were the 

 same. The unpublished counts (numbers per liter) reported by Starrett 

 (1972: 152) were: 7,000,000 to 233,000,000 in 1964; 6,800,000 to 

 108,000,000 in 1969; and 8,500,000 to 117,000,000 in 1970. Starrett 

 (1972: 152) reported that the decline in phytoplankton in the mid-1960's 

 was associated with a sharp decrease in the abundance of blue-greens , 

 while diatoms remained the dominant group. He suggested that the phyto- 

 plankton of the entire river might be limited by turbidity and the 

 synergistic effects of toxic metals. Williams (1964: 819) attributed 

 the high phytoplankton counts in the Illinois River to nutrient enrichment 

 and high calcium hardness. 



We do not feel that the reduction in algal populations is attribu- 

 table to toxicity, because diatoms are as sensitive as, or even more 

 sensitive than, other groups of algae. For example, Wong et al. 

 (1978: 479) found that a diatom was more sensitive to metal toxicity 

 than blue-green and green algae. Colbert et al . (1975: 59-60) found 

 few statistical differences between phytoplankton populations in the 

 lower Illinois and Mississippi rivers. Blue-green algae were approxi- 

 mately five times more abundant in the Mississippi than in the lower 

 Illinois, and phytoplankton evenness was slightly higher in the Illinois. 

 The causes of these differences are uncertain, and the physical-chemical 

 data reported by Colbert et al . (1975) for the two rivers do not show 

 any marked differences in phytoplankton. Colbert et al. (1975: 37-38) 



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