The annual variability in mayflies (Ephemeroptera), fingernail clams (Sphaeriidae),
and midges (Chironomidae) in six study areas of the Upper Mississippi River System
from 1992 to 1995 was examined. Spatial distribution is also discussed for these
organisms along with the Asiatic clam (Corbicula fluminea) and the zebra mussel
(Dreissena polymorpha). Sample allocation within each reach was based on a
stratified random design where strata were aquatic areas. No significant linear trends
across years were found in estimated reachwide mean number of organisms.
However, the overall test for differences in intercepts among study areas was
statistically significant (P < 0.05) for mayflies, fingernail clams, and midges. No
statistical difference in trend slopes among reaches were detected. In 1993, the
estimated mean density of fingernail clams in Pool 13 was 35 times that found in
other study areas. Overall, impounded aquatic areas and silt clay substrates supported
higher numbers of the select macroinvertebrates.
