A Comparison of Benthic Invertebrate Community Composition in the Mississippi and Illinois Rivers, Pool 26

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Abstract
Ponar, artificial substrate, and draft samples were collected to define benthic invertebrate community composition in Pool 26 at the confluence of the Mississippi and Illinois rivers. Illinois River Ponar samples produced primarily Chironomidae and Oligochaeta; Mississippi River Ponar samples were dominated by Chironomidae and Ephemeroptera. Mississippi River artifical substrates were dominated by Potamyia flava, Hydropsyche orris, and Chironomidae. Illinois River artificial substrates were dominated by Cyrnellus fraternus, Chironomidae, and Oligochaeta. Artificial substrates in the Mississippi River were colonized by twice as many taxa and 23 times more organisms than those in the Illinois River. Siltation appeared to be a major factor limiting artificial substrate colonization in the Illinois River. Composition of the drift was depth dependent and greatest numbers of organisms and taxa were collected in the two upper strata (0.3 m and 3 m). Mississippi River diel drift samples were dominated by Hydropsychidae, Ephemeridae, Chironomidae, and Chaoborus, while Illinois River drift was dominated by Chaoborus. Greatest drift densities generally occurred at 2200 and 0100 h.