Bug Hotels: How Insects Grade the St. Croix River's Water

Hester-Dendy samplers (left) sit at least 2 feet below the surface where the current is good. Insects settle on the plates as they would on a rock or log; back on shore the plates are rinsed to collect them (right). NPS / R. Damstra.

Counting the uncountable

Counting every aquatic macroinvertebrate — mostly insects — in the more than 200-mile St. Croix National Scenic Riverway would be hopeless. But Hester-Dendy samplers, or "bug hotels," collect a sample that shows the water quality at each spot.

• How long: the samplers stay in the river six weeks each summer.
• Since 2016: park staff have sampled four sites, each next to a freshwater mussel bed — an important park resource.
• Since 2019: the Great Lakes Inventory and Monitoring Network added seven more, also monitored for water quality.
• Together: eleven sites give a picture of the whole river's health, and the data feed Minnesota and Wisconsin's own assessments.

The ABCs of IBIs

Insects have long served as water-quality indicators. Most spend a year or more in the river as larvae and can't move far, so they integrate a site's conditions over time. From the number and kinds collected, biologists calculate an Index of Biotic Integrity (IBI).

• Stressors that lower the score: warm water, low flow, low oxygen, too much sediment, and possibly toxic pollution.
• Low IBI: one or more stressors at work. High IBI: healthier, cleaner water.

Average IBI scores at 11 sites, upstream (top; the first three on the Namekagon River) to downstream, over three to seven years; error bars show year-to-year variation. NPS.

Results: nearly every site shows insect communities in good or excellent condition. The two farthest downstream — Boom Sites #1 and #2, just above Lake St. Croix — score lowest, but have only three years of data. More years will show whether those scores are real, or whether the samplers suit sites so close to the lake. With enough years, IBI scores will show trends in river health at every site.

Not the only ones watching

• The Minnesota Pollution Control Agency's Large Rivers Monitoring Program found healthy insect communities along a 128-mile stretch of the St. Croix on the Minnesota–Wisconsin border in 2017–2018, and plans to repeat the work in 2028.
• Metropolitan Council Environmental Services has sampled Lake St. Croix at Stillwater, Minnesota, and Prescott, Wisconsin, where the lake drains into the Mississippi.

The Park Service's methods match both states' protocols, so its data can help decide whether stretches of the river are impaired.

More: the network's large rivers monitoring page and the St. Croix macroinvertebrate data.

Sources

Based on "Submerged Bug Hotels Tell a Story about Clean Water," Great Lakes Inventory and Monitoring Network, National Park Service; a work of the United States government in the public domain. It points to the Minnesota Pollution Control Agency's The St. Croix River: Study of the River's Health. The page's map of sampling sites is left out: its background appears to be a licensed Esri basemap. The source calls the index both "biotic" and "biological" integrity.