Annual status report, 1998: Macroinvertebrate sampling Sauer, J. 1999. Annual status report, 1998: Macroinvertebrate sampling. U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, Wisconsin, July 1999. LTRMP 99-P002. 23 pp. (NTIS PB99-164766) ABSTRACT In 1992, macroinvertebrate sampling was initiated in Pools 4, 8, 13, 26, the Open River reach of the Mississippi River, and La Grange Pool of the Illinois River as part of the Long Term Resource Monitoring Program. Long-term monitoring is needed to detect population trends and local changes in aquatic ecosystems. Mayflies (Ephemeroptera), fingernail clams (Sphaeriidae), and the exotic Corbicula species were selected for monitoring. Midges (Chironomidae) were added to the sampling design in 1993 and zebra mussels (Dreissena polymorpha) were added in 1995. Mayflies, fingernail clams, and midges, members of the soft-substrate community, were chosen because they play an important ecological role in the Upper Mississippi River System. Sampling was based on a stratified random design and was conducted at approximately 125 sites per study area. Mean densities of taxa were weighted by strata for extrapolation purposes. Pool 4 reported the highest estimated mean densities of mayflies and midges (208.9 and 253.0 m-2, respectively). Pool 13 had the highest estimated mean number of fingernail clams (150.1 m-2). Overall, the impounded areas, including Lake Pepin, and the contiguous backwaters tended to support the highest densities of mayflies, fingernail clams, and midges. Substrates with predominantly a silt clay constituent supported the highest mean densities of mayflies, fingernail clams, and midges. The presence or absence of Amphipoda, Bivalvia, Decapoda, Diptera, Gastropoda, Odonata, Oligochaeta, Plecoptera, and Trichoptera was also recorded. KEYWORDS Benthic aquatic macroinvertebrates, Corbicula, fingernail clams (Sphaeriidae), mayflies (Ephemeroptera), midges (Chironomidae), Mississippi River, zebra mussels (Dreissena polymorpha)