Large rivers, such as the Upper Mississippi River (UMR), are among the most productive freshwater ecosystems. Macroinvertebrate production is a primary energy pathway to riverine fish. Much of our existing understanding of macroinvertebrate standing crops in the UMR has been derived from sampling soft sediments, however, many macroinvertebrates are unable to colonize these highly disturbed habitats in the main channel. Current channel alignment structures (e.g., wing dams) and other natural hard substrates (e.g., tree snags) may provide stable, alternative habitats that support greater levels of macroinvertebrate production. The objective of this study was to investigate the colonization and secondary production rates for macroinvertebrates on stable substrates in the UMR. In the spring and summer 2006, Hester-Dendy artificial substrates (HDAS), consisting of 14 Masonite^® discs with a total area of 0.16 m², were placed at 3 locations (UMR River Mile 691.6, 693.3, and 699.8) along alignment structures. Three HDAS were collected from each location approximately once every 4 weeks. Using image analysis software, video microscopy and empirical area-to-weight relationships, over 8000 individual macroivertebrate colonists were identified and measured for size.

Results/Conclusions

Water temperature, an important factor affecting macroinvertebrate growth and production, varied during the study period, but not between sites. Filter-feeding caddisflies Hydropsyche orris and Cheumatopsyche campyla (Hydropsychidae:  Trichoptera) dominated the macroinvertebrate colonists in both number and biomass. Standing crop increased rapidly between June and July across all sites and peaked in August. Standing crop began to decline in September. Colonization rates were greatest, averaging 200.0 to 325.0 individuals m^-2 d^-1, in June and then monotonically declined to 37.5 to 56.3 individuals m^-2 d^-1 by September. Annual production rates for H. orris from UMR River Mile sites 691.6, 693.3, and 699.8 were 379.2, 350.9 and 246.6 g DW m^-2 yr^-1, respectively. These initial findings suggest that colonization and production rates for filter-feeding caddisflies on channel alignment structures and, potentially, natural, snag habitats, represent a much larger source of secondary production for main channel areas in the UMR than originally thought.