Winter Emergers in the Root River

The “Driftless Area” of southeastern Minnesota, southwestern ;Wisconsin, northeastern Iowa, and northwestern Illinois is one of the most spectacular and important landscapes of the entire Mississippi River. Towering bluffs, dissected by clear rivers and streams, make up a diverse biological and hydrological landscape that attracts homebuyers, recreational users, and scientists studying the unique features of the area.

Groundwater dominated trout streams in southeastern Minnesota’s Driftless Area, such as the Root River, offer unique opportunities for researchers interested in aquatic ecology. During the cold and snowy Minnesota winters, while most people are content spending the day sipping hot chocolate in front of a warm fireplace, members of the University of Minnesota’s Chironomid Research Group can be found spending the day in a very different manner. While most researchers have wrapped up their outdoor field work by the time the snow starts to fly, this cold-hardy group is just getting started with some exciting research projects that takes place in various groundwater dominated streams that are found throughout central and southeastern Minnesota. These streams are unique in that they remain, for the most part, ice-free throughout the winter due to the moderating effect of groundwater inputs. This allows members of the Chironomid Research Group to gain insight into the dynamics of winter-emerging aquatic insects, specifically the chironomids.

Chironomids, more commonly known as “midges,” are a family of small-sized flies who live out their larval and pupal stages in lakes and streams. These insects, which often comprise over 50% of the aquatic insect community, are incredibly diverse and widespread. They have adapted to live in a variety of climatic and water conditions. Some species even occur in streams fed by glacial ice-melt in high arctic and alpine areas, and one species occurs in the Antarctic, where it is the largest land animal to persist year-round on the continent! In Minnesota many groups of these midges (and the researchers that study them) have evolved to resist and flourish during even the coldest of our winters, when most other insects (and entomologists) are hiding away and awaiting the warmth of spring. If you walk the banks of just about any trout stream during the winter, chances are that you have seen many of these flies on the snow.

Perhaps one of the most interesting aspects of these winter-emerging insects is how the presence of these insects relates to other communities inhabiting the stream. For example, fish populations living in streams with a high abundance of winter emerging insects will be able to grow throughout the winter months due to the presence of this food source. Minnesota has one of the most impressive winter trout fishing programs in the United States, and the impact of these flies on trout growth pleases many winter trout fisherman who are satisfied with only the biggest catches.

Of particular interest to these scientists is how climate change will affect these winter-adapted species. Many of these midges seem to be specially adapted to develop only after the water has dropped below a certain temperature, suggesting that increases in temperature due to climate change scenarios may have a major impact on this community. If midge species are not able to adapt to these new conditions, this could lead to northward range shifts or local extinctions of the insects, which would result in further implications for fish populations. Through their research, the Chironomid Research Group hopes to paint a better picture of how the midge community will be affected by a warming environment and how these changes will transfer through food chains to affect other members of the aquatic community.

by Alyssa Anderson

Root River, Fillmore County, Minnesota

Winter Emerging Chironomidae, walking on snow.

Root River, Fillmore County, Minnesota


Resistance and Resilience of Winter Emerging Chironomidae

Climate change has become an important issue in our world as evidence of its existence is becoming increasingly obvious. Recent climate predictions indicate that the Midwest will exhibit a 3% increase in average annual precipitation and the intensity of these events is expected to increase (Trenberthet al. 2007). These changes may have extensive consequences on the composition and quality of aquatic insects in the stream. As all good fly-fishermen know, the aquatic insect community is vital in contributing to the diet and growth rate of trout. Diets rich in energy (insects!) will produce the most productive (and plump!) trout. And this is where the interest and expertise of the Chironomid Research Group comes into play.

Graduate student Alyssa Anderson and Dr. Len Ferrington are trying to determine how resistant or resilient chironomids, or midges, are to increased stream flow rates by studying the response of winter-active midges to the extreme flooding events that occurred in southeastern Minnesota during the August of 2007. In aquatic ecology, resistance is defined as the ability of the community to deflect or absorb a disturbance with minimal change, whereas resilience is defined as the ability to quickly recover from a disturbance. Anderson and Ferrington gathered data throughout the winter of 2007-2008 to determine the types and abundance of midges present in a number of trout streams in southeast Minnesota. They then compared these data to results obtained in previous years in order to determine if midges are able to either resist or quickly bounce back from the extreme conditions brought about from the flood event. The ability of the midge community to recover from these types of events may have severe implications for both growth and stability of trout populations, especially if these aquatic insects are not able to quickly recover to pre-flood population levels. Their results have indicated the midge community is resistant to the effects of extreme precipitation events, therefore, extirpation of this potential prey species is not a concern for trout


Trenberth, K. E., R. D. Jones, P. Ambenje, R. Bojariu, D. Easterling, A. Klein Tank, D. Parker, F. Rahimzadeh, J. A. Renwick, M. Rusticucci, B. Soden, and P. Zhai. 2007. Observations: Surface and atmospheric climate change. In S. Solomon, D. Quin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor and H. L. Miller (eds.), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA

Damage caused by the August 2007 Flood in Southeast Minnesota

Winter Sampling for Midges at the Root River

Collecting Samples of Winter Emerging Chironomids

One of the easiest ways to assess which midge species are emerging from trout streams is by collecting and identifying their pupal exuviae, the skins that are cast by the flies as they complete the transition from pupae to adult. The collection method is fairly simple. The cast skins are collected by successively dipping an enamel pan into the water downstream of areas that are known to have accumulations of pupal exuviae, such as areas of accumulated foam or debris. All of the contents collected in the pan are then passed through a sieve. This process is repeated for the duration of the sampling period, which typically lasts ten minutes. The contents of the sieve are then transferred to a labeled sample container and preserved in ethanol for later identification (See Ferrington, et al. 1991 for further information). During most collection regimes, scientists collect every three hours during an entire 24 hour span, rising at midnight, 3 AM and 6 AM often in sub-freezing temperatures to dip their basins in the stream.

Sampling Methods

Chironomid Pupal Exuviae

Areas of foam accumulations are prime collection sites for
chironomid pupal exuviae.

Preserving Collected Material


Ferrington, L. C. J., M. A. Blackwood, C. A. Wright, N. H. Crisp, J. L. Kavanaugh, and F. J. Schmidt. 1991. A protocol for using surface-floating pupal exuviae of chironomidae for rapid bioassessment of changing water quality. In Anonymous Proceedings of the Vienna Symposium, Vienna.

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