Effects of dredge material placement on benthic macroinvertebrates of the Illinois River

Since the 1930s, dredge material has been removed from the Illinois River and placed along the main channel border in shallow depths to maintain a 2.7 m deep main channel for commercial navigation. Placement of this material changes the sediment composition from primarily silt/clay to primarily sand, and it buries pre-existing benthic invertebrates. During 1997 and 1998, the benthos of an 125 km reach of the middle Illinois River (La Grange Reach) was studied by extracting 1065 Ponar samples from randomly-selected sites which had never received dredge material, received dredge material one year previous, or received dredge material during the current year. Although total numbers of macroinvertebrates collected was lower in 1998 than in 1997, relative abundances of eight targeted taxa were highly similar between years. Chironimidae were most abundant and comprised >66% of all macroinvertebrates collected both years. Differences in densities of Chironomidae, Ephemeroptera, Sphaeriidae, Corbicula fluminea (Muller, 1774), Dreissena polymorpha (Pallas, 1771), Odonata, and Gastropoda among the three classes of dredge material placement were all significant (P< 0.05). For all taxa, densities were highest at sites that had never received dredge material; and, for all taxa except Chironimidae, densities were lowest at sites that received dredge material during the current year. No significant recovery by macroinvertebrates was noticed on dredge areas of this reach after one year (P>0.05). Future operations to maintain a channel for navigation should consider preexisting densities of macroinvertebrate taxa. Because benthic macroinvertebrates are an important component of the food web and shifting sand does not support diverse macroinvertebrate communities, strategic placement of dredge material by avoiding islands or other areas of high macroinvertebrate diversity could improve overall system productivity and biotic integrity of large river-floodplains.