Recovery of stream invertebrates after catastrophic flooding in southeastern Minnesota, USA

Abstract

Widespread catastrophic flooding struck coldwater streams in southeastern Minnesota in August 2007, drastically scouring and altering stream channels and their biota. Post-flood invertebrate abundances and assemblages in streams in and near Whitewater State Park were compared to data collected from these same streams prior to flooding. Flooding reduced invertebrate densities by 75–95% in some streams. Taxa richness was reduced by 30–70%, and assemblage structure was reduced to poor and very poor levels in first- and second-order streams but not in the larger (fourth-order) system. Ephemeroptera, Plecoptera, and Trichoptera were especially impacted in small streams, and amphipods were virtually eliminated from both large and small streams. Densities of some invertebrate groups recovered within months of the flood, but others were still recovering after nearly two years. Taxa richness and community structure returned to pre-flood levels at most sites within one year, but total densities at many sites remained below long-term averages 22 months post-flood. Invertebrate assemblages were impacted differently by flooding in small versus large streams, resulting in differing patterns and degrees of recovery.