Size distribution of aquatic invertebrates in two prairie wetlands, with and without fish, with implications for community production

1. We compared the size distribution of aquatic invertebrates in two prairi e wetlands, one supporting a population of fathead minnows and the other fi shless. Both wetlands were sampled in three depth zones on three dates, all owing assessment of temporal and spatial variation. 2. We determined biomass of aquatic invertebrates in 17 log(2) size classes , and used these data to develop normalized size spectra. We also coupled s ize distributions with an allometric model to estimate relative production at the community level. 3. The composition of the invertebrate communities differed greatly between sites, and invertebrate biomass was higher in nearly all size classes in t he fishless wetland. Intercepts of normalized size spectra were significant ly different between wetlands, but slopes generally were not, indicating di fferences in standing-stock biomass but similar size structures between the two invertebrate communities. Higher standing-stock biomass in the fishles s wetland resulted in higher relative production per unit area, but similar size distributions resulted in similar mass-specific production (P/B) betw een wetlands. 4. Our results indicate that invertebrate communities in prairie wetlands m ay have relatively consistent size structures in spite of large differences in community composition and standing-stock biomass. We hypothesize that t he observed differences are because of predation by the minnow population a nd/or differences in the macrophyte communities between the two sites. Howe ver, the relative importance of macrophytes and fish predation in structuri ng invertebrate communities in prairie wetlands is poorly known.