river traffic would be expected to have the greatest impact. Since no 

 one knows what the benthic populations would have been in the absence 

 of river traffic, their conclusion should be regarded as tentative. 



It is also possible that changes in the chemistry of the water and 

 sediment in the Mississippi River between July and September, 1974 in- 

 fluenced the benthic organisms. For example, the dissolved oxygen con- 

 centration increased significantly, while the turbidity and settleable 

 solids declined significantly. Both trends would favor benthic organisms. 

 Benthic organisms live in or on the sediment, so the chemistry of the 

 sediment would be expected to influence them. The concentration of cer- 

 tain toxicants, such as ammonia, zinc, and cyanide, declined significantly 

 between July and September (Colbert et al., 1975: Table 6). More infor- 

 mation on the relationship between physical-chemical factors and benthic 

 organisms needs to be obtained before one can make a rational choice 

 among the above alternative hypotheses. 



Importance of Benthos as Food for Fish and Wildlife. Studies have 

 been conducted by Iowa State University and the Illinois Natural His- 

 tory Survey on the use of benthic organisms by fish and waterfowl in Pool 

 19. The fingernail clam is an important food item in the diets of many 

 fishes and diving ducks in Pool 19. Carp, gizzard shad, smallmouth buf- 

 falo, white sucker, and black bullhead feed extensively on fingernail 

 clams (Jude, 1968: 227, 228, 229). Thompson (1969) estimated a daily con- 

 sumption of 229 grams of sphaeriids (blotted with shell intact) per 

 diving duck on Pool 19. This rate would have amounted to a sphaeriid 

 harvest of over 2 million kg by ducks during both the spring and fall 



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