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Upper Midwest Environmental Sciences Center

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Macroinvertebrates

 

  • Macroinvertebrate (see photo-invertebrates visible to the naked eye) monitoring by Wisconsin's LTRMP Field Station consisted of one episode of stratified random sampling in May or early June to sample five target soft-substrate invertebrates (mayflies, fingernail clams, midges, Asiatic clams and zebra mussels) with ponar dredges from 1992-2004.
  • One-hundred-twenty-five sites throughout Pool 8 were sampled randomly.
  • About 25 additional fixed historical sites were also sampled each May; the exact number sampled depended on water level.
  • Presence/absence data were noted on nontarget macroinvertebrates incidentally recovered during sampling such as mollusks, mussels, stone flies, dragon flies, caddis flies, scuds, oligochetes, and others.
  • Macroinvertebrate monitoring program wide was discontinued in 2005 due to budget reasons.

More specific information needed to interpret LTRMP invertebrate data can be found at the following link: (invertebrate data format file.) Invertebrate data can be downloaded in text format at this site as well (invertebrate data browser).

Mayfly swarm mayfly

photo credit: Dr. Calvin Fremling

Mayfly larva of the genus Hexagenia filter out tiny particles of food for a year in U-shaped burrows they make in the mud of the river bottom. In late June to early July, they emerge as winged adults and fly in huge swarms to mate.

The adults are often attracted by lights on bridges or at gas stations and can fly several miles inland, commonly creating a slippery mess as they pile up and die on the pavement below the lights. At times they have been so thick on bridges that they had to be cleared with snowplows.

Mayflies are sensitive to gross organic pollution and their presence is good news ecologically because it means that gross organic pollution such as sewage is not present in large amounts. Because they live in the sediment for most of their lives, mayflies have also been extensively studied as potential accumulators and indicators of trace contaminants such as heavy metals and PCBs.
Invert sampling (photo)
Invertebrate sampling in the LTRMP involves lowering a ponar dredge to the bottom of the river and scooping up the soft bottom (usually sand, silt, clay, or a mixture). Sand, silt, and clay are then washed through a screen, leaving the aquatic insects, clams, mussels, and snails behind on the screen. These are identified. Target organisms (mayfly larva, midge larva, fingernail clams, zebra mussels, and Asiatic clams) are counted, and presence/absence is noted on the others. The organisms screened are then returned to the river.

Ponar dredge

The jaws of the ponar dredge have been opened and mud grabbed from the bottom of the river is being rinsed into a wash screen. The mud will pass through the mesh of the wash screen, leaving the inhabitants of the river bottom behind.
Invert sampling

Along with the 125 random sites stratified by type of aquatic area, we also sampled 20-25 historical fixed sites that were sampled prior to the beginning of the Long Term Monitoring Program. Sedimentation in the backwaters, presumably due to water flow dynamics involved with over 60 years of impoundment, has filled in many of the historical sites to the extent that it is difficult or impossible to navigate a boat to them. Here, the La Crosse field station invertebrate sampling is stalled when shallow waters make it impossible to motor to a historical site and crew members must get out and push. Pushing boat (photo)

Sunset boat (photo)
In autumn, LTRMP field stations cooperate with state DNR and USGS and US Fish and Wildlife staff to sample invertebrates in areas where waterfowl are likely to congregate along the upper Mississippi River. This helps them predict the food supply and manage refuge areas closed to waterfowl hunting. Jeff Janvrin of the Wisconsin DNR's Mississippi/Lower St. Croix Team took this photo of the La Crosse Field Station's boat rigged for invertebrate sampling in Pool 11, north of Dubuque, Iowa during one of these fall sampling trips.


La Crosse Field Station Team Leader
Telephone: (608) 781-6360
Fax: (608) 783-6066

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Page Last Modified: March 13, 2014