Winter-emerging chironomidae (Insecta: Diptera) in Minnesota trout streams

Abstract

The Chironomidae (Diptera) is among the few aquatic insect families with species that are reliably able to grow, develop, and emerge as fully functional adults throughout the cold winter months that characterize the midwestern United States. However, few studies document their role in trout streams during this time of year. The intent of this dissertation is to enhance the field of Chironomidae research by focusing on the winter dynamics of these insects, including their response to severe flood events, their incorporation in the winter diet of trout, and the morphological and molecular description of three new cold-adapted Chironomidae species. The resistance and resilience of the winter chironomid community was assessed following a severe flood that impacted many streams in southeast Minnesota during the late summer of 2007. Methodology included collections of chironomid surface-floating pupal exuviae from 18 southeast Minnesota streams; streams were selected to include representation of localities that incurred moderate to extreme levels of flooding disturbance. These data were compared to data from the same localities during prior winters. Significantly more taxa emerged during the winter after late-summer flooding as compared to historic collections, and the number of species emerging in winter was positively correlated with rainfall severity, indicating the winter-active Chironomidae are resistant to late summer spates. This indicates that chironomids are an available winter food resource to stream trout recovering from devastating floods. In attempt to examine the importance of Chironomidae and other winter-active aquatic insects to the winter diet of brown trout (Salmo trutta L.), stomach contents were collected from trout inhabiting three southeast Minnesota streams; these data were compared to organisms collected in the stream drift and benthos. Results indicated that trout diets differ by stream and by fish size, with larger trout feeding heavily on Trichoptera and Physella, and smaller fish relying more on Gammarus and Chironomidae larvae. Stomach contents were more similar to the benthos than the drift, indicating a greater reliance on benthic feeding during winter. Trout in all streams selected Trichoptera and Chironomidae over other prey, and appeared to maintain sizeselective predation throughout winter. Winter-emerging Chironomidae appear to enhance the trout diet in all streams, and chironomid larvae were particularly dominant in the diet of one trout population. Three new chironomid species within the genus Micropsectra, a genus commonly found in the winter trout diet, were discovered from a study of five Minnesota streams. These species, Micropsectra neoappendica, n. sp., Micropsectra penicillata, n. sp., and Micropsectra subletteorum, n. sp., were described using morphological and molecular methods, along with one additional species, Micropsectra xantha Roback, which was redescribed. Two of the new species initially appeared identical to species known from the Palearctic, however molecular data indicated they are genetically distinct. Subsequently, reexamination of morphological characters revealed slight, but consistent diagnostic differences. These results emphasize the importance of using molecular tools in conjunction with traditional morphological techniques when studying Chironomidae diversity, especially when relying on diagnoses from other regions. The culmination of these studies sheds light on the dynamics of winter-emerging Chironomidae in southeastern Minnesota trout streams, their distribution and abundance, and the larger role they play in stream communities.