Winter diet of brown trout Salmo trutta in groundwater-dominated streams: influence of environmental factors on spatial and temporal variation.

Winter diet composition of brown trout Salmo trutta was quantified from November to March in 35 temperate groundwater-dominated streams in south-eastern Minnesota, U.S.A., in relation to stream physical characteristics including drainage area, channel slope and influence of groundwater on stream thermal regime. Aquatic invertebrates made up the majority of S. trutta diet in all streams and sampling periods and individual S. trutta typically had consumed 30 or more prey items at each sampling event. Differences in diet composition were greater among streams than between sampling periods within a stream, with Gammarus spp., Brachycentrus spp., Glossosoma spp., Chironomidae and Physella spp. the most common taxa. Landscape-scale stream characteristics were not significantly associated with S. trutta consumption or diet composition. Winter was period of significant activity in groundwater-dominated streams, as S. trutta fed on a variety of aquatic prey taxa highlighting the importance of winter base-flow in moderating S. trutta populations in seasonally cold catchments.

Winter growth, development, and emergence of Diamesa mendotae (Diptera: Chironomidae) in Minnesota streams.

Diamesa mendotae Muttkowski is commonly observed emerging during winter at low air temperatures from Minnesota streams, but little is known about the winter dynamics of this and other winter-emerging aquatic insects. Biweekly collections of surface-floating pupal exuviae indicated that this species emerged from October through May, when water temperatures were less than approximately 10 degrees C. Emergence occurred continuously through winter, with a lull during January and February. Development of larvae from in situ growth enclosures supported emergence data and indicated that the reduction and halt of emergence in the spring was related to increasing water temperatures (> 10 degrees C), which were unsuitable for the growth or survival of the larvae. Development continued through January when water temperatures were at their lowest for the study stream and therefore did not explain the mid-winter lull in emergence. Growth rates of D. mendotae were not greater than other chironomid taxa at similar temperatures, indicating that lower critical thermal limits for growth allow this species to dominate ground-water influenced streams during the winter in Minnesota. The results of this study show that D. mendotae is well suited for growth and development at low temperatures and provides an assessment of important factors that regulate this species at low water and air temperatures.