Effects of flooding on resistance and resilience of winter emerging Chironomidae in Southeast Minnesota trout streams

Disturbances associated with rainfall events are common in stream systems. Many aquatic invertebrates have evolved mechanisms to resist flooding, while others exhibit high resilience following this type of disturbance. Extreme flood events, however, may have severe consequences to resident invertebrate species. In August 2007, much of southeastern Minnesota experienced extreme flooding, with some areas receiving over 15” of rain in 24 hours. SE Minnesota is refuge to 188 trout streams that are of high economic value for anglers and entrepreneurs dependent on income from sport fishing. This rainfall event imposed severe stress on aquatic communities inhabiting these streams. Chironomidae typically comprise over 50% of aquatic insects, and are a major protein source of trout.  Bi-weekly collections of chironomid pupal exuviae were made from 18 trout streams in southeast Minnesota during winter 2007-2008 in order to quantify emergence; streams were selected to provide an equal representation of moderate (1-4”), heavy (4-8”), or extreme (8+”) rainfall categories. All streams had historical winter emergence data, so resistance and resilience of winter emerging populations to severe flooding events could be quantified. Our findings show that winter emerging chironomids are highly resistant to the stress imposed by extreme precipitation events.