Winter Invertebrate Community Dynamics in Groundwater-Fed Streams of Southeastern Minnesota

Groundwater-fed streams, which remain cold in summer but ice-free in winter, provide ideal habitat for ultra-cold stenotherm insects and trout. Previous studies of these insects have focused on their thermal tolerance limits; however, their role within the invertebrate community is not well-established. We documented invertebrate community composition and cold stenotherm emergence in 12 groundwater-fed streams in southeastern Minnesota by collecting samples on three occasions throughout the winter of 2010-2011. Hess samples were used to evaluate winter invertebrate community composition and surface-floating pupal exuviae collections were used to study the emergence patterns of cold-adapted insects. Both invertebrate community composition and ultra-cold stenotherm emergence were dynamic throughout the winter in each stream. Additionally, invertebrate communities differed significantly between streams, with early-winter average abundances ranging from 855 to 7874 individuals per square meter of riffle habitat. Average invertebrate abundance was positively related to stream thermal stability. Winter emergence patterns also varied between streams, likely as a result of different groundwater inputs to each stream. This range of thermal buffering causes each stream in southeastern Minnesota's karst landscape to have unique winter invertebrate community dynamics and emergence patterns.