The St. Croix River is home to an amazing array of itty-bitty insects called Chironomidae, according to a recent study. Researchers have found hundreds of species in the upper river, with analysis in progress on the rest of the waterway.

The new report from the National Park Service and University of Minnesota describes an astounding richness of midges in the uppermost St. Croix. The peer-reviewed paper is based on samples collected more than a decade ago.

The findings have gotten a lot of attention.

“People around the world are fascinated by the number of species in the St. Croix,” says co-author Len Ferrington of the University of Minnesota. “This publication has been a real hot topic among people working in aquatic insects across the world.”

In just 10 miles of river below Gordon Dam, the scientists found evidence of 252 species of midges. It’s the highest number of species identified in any river in North America, says Ferrington. Other surveys of streams and rivers usually find at most about 150 species.

The species found in the St. Croix include several that were not known to live in the region, and even several possible new species.

No one can say for sure why the St. Croix River is so rich with midges, but there are a few theories.

“Part of it is overall good water quality, good habitat quality, and not a lot of sediments that are smothering or causing problems with oxygen levels at sediment water interface,” Ferrington says. “In the upper St. Croix, you get fairly good diversity of [river bed] types from boulders down to sand.”

These minuscule insects commonly called midges live in the water until transforming into flying adults to reproduce. The aquatic larvae feast on algae growing on the rocks and organic material that flows into the river from land. Minnows in turn eat the midges, and popular game fish like walleye, smallmouth bass, and muskellunge eat those minnows.

Without midges, there simply wouldn’t be as many big fish. They are a key link in the food chain.

There is a lot yet to learn about Chironomidae. There are many species that haven’t even been described by science, questions about what lives where, their life cycles, and more.

The ongoing studies focused on the St. Croix are finding some local answers, and raising awareness of yet another way the river is so special.

The study has taken a lot of work over many years. The initial field surveys were conducted in 2006 and 2007, thanks to a grant from the National Park Service and assistance from recently-retired Chief of Resource Management Byron Karns.

The small amount of federal funding allowed the team to intensively collect samples up and down the river during one full open-water season. They visited a total of 17 sites from the Gordon Dam to the mouth of the St. Croix at Prescott, once a month, and collected samples from along both banks.

“This was a once in a lifetime to work on that river,” Ferrington says.

They retrieved more than 5,000 specimens of the skin left behind by pupa when they transform into adults, and remains floating on the water. Many of those cases can be identified as unique species.

Since then, a former graduate student of Ferrington’s who is now a National Park Service entomologist was able to identify the collected specimens in his spare time. The effort was well worth it.

“I’m always excited about the little details that pop out of a study like this, especially when we can confirm range expansions into a new part of the continent,” said Alex Egan, who now works for the National Park Service’s Great Lakes Network, based in Ashland, Wis. “Chironomids can be very hard to identify to species, so most studies are not able to contribute to species ranges.”

Working with a microscope, Egan carefully identified the samples, many of which he was able to identify as a specific species.

Egan says at least one of the species he identified should allow researchers to finally describe it as a new type of Chironomid. While other scientists had previously described its adult form, but had not positively identified a pupal case before. The St. Croix River samples should allow him to do that now, making an important contribution to scientific knowledge.

Ferrington also says he hopes to help describe a couple new species from the samples in the next year.

The study could also help protect the tiny organisms, which play such an important part in the food web. Ferrington points out it is valuable to have data available about what species live where in the event of a pollution incident. He points to the possibility of a chemical spill — or climate change.

“As a baseline we’d want to know what taxa is there, so one could gauge if there was an unforeseen spill,” Ferrington says.

Meanwhile, ongoing environmental threats pose a challenge to the midges’ long-term survival. Many of the species need cold water with lots of oxygen. Global warming could affect those characteristics of the St. Croix’s water. Egan lists road salt and the increases in chloride in lakes and rivers that it can cause as another threat.

The larval midges living in the water depend on leaf litter and other organic material that ends up in the river for food. Ensuring vegetation remains in the “riparian zone” (along the shoreline) can help ensure nothing disrupts that food source. The loss of vegetation can also increase erosion that smothers midges and the habitat they need to survive.

“In the big picture, protecting the riparian zone is important,” Ferrington says. “Number one, we want that organic matter coming from streamside vegetation to continue serving as a food resource for them. Along with that, is if we have healthy riparian systems, that protects against erosion and prevents fine sediments.”

The 2007 sampling effort included the entire St. Croix National Scenic Riverway, from Gordon Dam to the Mississippi River. Egan will continue working through samples from the rest of the river, and he and Farrington plan to publish more papers describing the diversity found throughout the St. Croix.

Reference

Egan, A. T., & Ferrington, L. C. (2019). Chironomidae of the Upper Saint Croix River, Wisconsin. Transactions of the American Entomological Society, 145(3), 353-384. https://doi.org/10.3157/061.145.0307