Timing is everything

Flash floods led to discoveries about trout food web

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Alyssa Anderson harvesting midges from a trout stream in southern Minnesota.

By Martin Moen

June 19, 2008

Luck and preparation are paying off for Alyssa Anderson. Since junior high in Lake Crystal, Minnesota, she's learned all she can about aquatic insects. Anderson's passion for the field earned her awards and travel to science competitions in California, Florida and the United Kingdom before she came to the University of Minnesota in 2007 seeking a Ph.D. She arrived just in time to study Minnesota's greatest rainfall event.

In mid-August a stalled weather front dumped buckets of rain in southeast Minnesota. The storm set a new National Weather Service--a one-day state record of 15.1 inches of rain, with an unofficial storm total in some areas of nearly 21 inches of rainfall. The storm rerouted several of the region's 188 trout stream channels and forever changed others. The storm's effect on the life within those streams was unknown.

The storm, though, created an opportunity for Anderson and entomology professor Len Ferrington to learn more about how aquatic insects react to summer floods. The focus of their work was 15 species of winter-emerging Chironomids, commonly called midges, are a family of small flies and an important source of protein for trout.

The fear was that the storm had given the trout a double-whammy. Southeastern Minnesota is home to three species of trout: brook trout (native to the area), brown trout, and rainbow trout. Many of them were likely displaced by the flooding and realignment of streams, and those that did survive would not have enough food over the winter because the midge population also had been affected.

Their preliminary findings suggest that the storm lowered the overall quantity of winter-emerging midges only slightly in the flood-ravaged streams. The surprise, Anderson and Ferrington found, was that the rains may have helped diversify the species of midges.

"In the hardest-hit streams we saw the greatest diversity of midges," Anderson says. "We've even found a few species that were never collected in previous winters. The August flood may have given these species an opportunity to colonize and fill niches left open by warm-weather midges that suffered greater losses from the flood."

Anderson says the diversity of species is a great sign.

"This indicates that even the streams that were most severely affected are on the road to recovery and are producing a sustainable amount of food for the fish population inhabiting these streams," she says. "In August these midges are in their larval stage and hibernating in the stream beds. So the midges were harmed only where the stream beds were scoured out or exposed because the stream changed its path."

The results include historical data on 18 trout streams, some of which experienced extreme, moderate, or no ill effects from the storm. For three months this past winter, Anderson, Ferrington, and others made weekly visits to the streams in their study.

"It takes a special person to step into a fast-flowing stream when the air temperature is below zero," says Ferrington. "I think Alyssa is as cold-adapted as I am--or as crazy about science. Her enthusiasm and eagerness to learn are outstanding."

Their findings help explain the moderate success anglers found this spring during Minnesota's trout season opener. Further studies next winter will help determine the long-lasting effects of the August 2007 storm.

More information about this project is available at the University of Minnesota Department of Entomolgy.

Edited from Solutions, a publication by the College of Food, Agricultural, and Natural Resource Sciences