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.
