Winter invertebrate dynamics in groundwater-fed streams in southeastern Minnesota, USA

Abstract

Southeastern Minnesota is characterized by Karst geology and trout streams. These groundwater-fed streams remain cool in summer and ice-free in winter, providing ideal habitat for trout and cold-adapted insects. Previous studies of winter-active insects have been localized or laboratory-based; however, the broader winter invertebrate community and its relationships to groundwater input are not well established. The goals of this research were to (1) assess the emergence patterns of winter-active chironomids (Diptera); (2) evaluate the effect of groundwater on the voltinism of the chironomid species Diamesa mendotae Muttkowski; (3) assess overall winter invertebrate community composition and abundance; and (4) describe the winter benthic chironomid community. Invertebrates were collected from 36 groundwater-fed streams over three winters (2010 - 2013). Surface-floating pupal exuviae (SFPE) collections were used to study the emergence patterns of cold-adapted chironomids, and Hess samples were used to evaluate winter benthic invertebrate composition and abundance. A total of 14 chironomid genera emerged from December through February; an additional 16 genera emerged in March. D. mendotae was the most commonly encountered and abundant winter-emerging chironomid, and emerged throughout the winter. Analysis of stream thermal regime indicated that D. mendotae complete multiple generations in a single winter in most streams. Benthic invertebrate communities were dominated by few taxa, with three chironomid genera (Diamesa, Orthocladius (Orthocladius), and Pagastia), two mayfly genera (Baetis and Ephemerella) and one caddisfly genus (Hydropsyche) comprising over 60% of individuals collected. Overall abundance was highest in thermally stable streams. Chironomids were abundant and diverse, with the winter-active species Diamesa mendotae dominating the community. We conclude that groundwater-fed streams (sensu Krider et al. 2013) in southeastern Minnesota support large, winter-active invertebrate communities, which are ecologically important to brown trout.