found that snails ( Physa sp.), which were reared in the laboratory 

 under conditions designed to reduce their exposure to pesticides as 

 much as possible, rapidly accumulated dieldrin when exposed in cages to 

 Illinois River water at mile 87 for a period of 8 days in August 1974. 

 The dieldrin content of the snails increased significantly within 24 

 hours, and after 8 days the dieldrin content had increased from 0.1797 

 ppm wet weight, whole organism, to 0.8156 ppm. The increase in dieldrin 

 content of the snails showed no signs of reaching a plateau within 8 

 days, so the ultimate equilibrium concentration of dieldrin in the 

 snails was undetermined, but certainly much higher than 0.8 ppm. 



Possible Effects of Barge Traffic on Benthos in the Illinois River. 

 Colbert et al. (1975: 95) felt that the increase in benthic populations 

 in the main channel of the Mississippi between a high-flow period in July 

 of 1974 and an average-flow period in September 1974 indicated that 

 barge traffic did not have a dominant influence on benthic organisms. 

 Colbert et al. reasoned that the detrimental impacts of barges, if any, 

 should be greatest under low-flow conditions. The data of Colbert et 

 al. (1975: Table 6) for the benthic organisms in the Illinois River show 

 no statistically significant differences in the populations between July 

 (a period of high flows) and September (a period of low or average flows), 

 except for a significant decline in the number of clams. The Illinois 

 is a much smaller river than the Mississippi (the channel is shallower 

 and narrower) , and it is possible that barge traffic has a greater im- 

 pact on benthic populations in the Illinois during low flows than in 

 the Mississippi. 



It is noteworthy that the turbidity and settleable solids were 

 greater in the Mississippi than in the Illinois during high flows, but 

 that both factors declined to a greater extent in the Mississippi than 

 in the Illinois during low flows. The turbidity and settleable solids 

 reflect the amount of suspended sediment in the river, and it may be that 

 boat traffic more easily resuspends sediment in the Illinois during low 

 flows than in the Mississippi. The disturbance of the bottom, or the 

 resuspended sediment itself, may have detrimental effects on benthic 



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