Combining Sonar-Based Habitat Quantification with Macroinvertebrate Sampling Reveals the Importance of Underrepresented Habitats in a Large River

Identifying sources of energy that support large-river metazoans is important for management of threatened large-river species. A trophic approach in which food resources for the Pallid Sturgeon Scaphirhynchus albus and other abundant competitors are quantified may be vital towards improving management of this endangered species. We examined how macroinvertebrates, a dominant prey item for young Pallid Sturgeon, vary in abundance, biomass, and community structure at different benthic habitats in the upper Missouri River. Side scan sonar was used to quantify the proportions of benthic habitats at two reaches between Fort Peck Dam and Lake Sakakawea in the upper Missouri River, MT. Both reaches contained predominantly sand habitat (90-93%), with small patches of large-rock habitat (LRH) (2-3%). Estimates of total macroinvertebrate abundance and biomass were significantly higher on LRH (P<0.01 and P<0.01, respectively). Additionally, macroinvertebrate communities on LRH, which were dominated by Hemiptera, Ephemeroptera, and Trichoptera, were significantly different from communities in sand, which were primarily chironomids in order Diptera (analysis of similarity P<0.05). Our results suggest that while LRH may only be a small proportion of the total habitat, it supports an abundant and distinct macroinvertebrate community that may be important for consumers at higher trophic levels.