Macroinvertebrates

This chart shows the relative pollution tolerance of select types of macroinvertebrates. The EPT taxa have some of the lowest tolerance values, making them indicators of high quality water. (Courtesy of Beaver Water District, Lowell, Arkansas).

Editor’s Note: This is a column on area watersheds by Blyden Potts and guest columnists to spread awareness of the area’s tributaries and the efforts of area volunteers to keep them clean.

 

Saturday I attended a field lab called “Macroinvertebrates 101.” Most participants had some experience collecting and identifying invertebrates, so it was more of a refresher than an intro class. Invertebrates are animals without a backbone. Macroinvertebrates are those large enough to see, unaided by magnification.

The idea is to collect sample macroinvertebrates from a waterway. A healthy stream will have a diversity of species.

Which species are present varies by the local ecology of the stream section, but also by water quality.

Some aquatic invertebrates are much more sensitive to pollution. Others can survive mild or moderate pollution, and a few are pollution tolerant. Scientists use this as a natural index to assess how clean a stream is and has been over the lifespan of the organisms.

Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies), referred to collectively by the acronym EPT, are especially sensitive to pollution. Their abundance in a stream indicates very high water quality. They are also among the core prey of trout, one reason trout do best in very clean water.

Scientists have sampling protocols and use microscopes to better see and identify what they collect. With a microscope, I was able to identify a tiny Planarian I would not otherwise have been able to identify, but many macroinvertebrates (e.g. crayfish, bivalves, large snails, aquatic worms) can be identified with the naked eye, and others (e.g. mayflies, stoneflies, damselflies) with nothing more than a magnifying glass.

A simple version of this, without sampling rules or microscopes, can be a fun and educational activity for anyone, especially kids. Wear clothing and footwear suitable for wading in your local stream. You will need a net with a fine mesh, and a few plastic containers to put what you collect in. White ones provide the best visual contrast. Wade in the stream. Scrape your net along the bottom or the surface of rocks, or through aquatic vegetation or leaf litter, moving it in upstream direction, against the current. Try not to get much sediment.

Empty the net into a container or pick critters out of the net, with tweezers or fingers, and place them in the containers. Then observe, and try to identify, what you have found.

You will want a reference book or other identification guide. Many quick guides are available online. Stroud Water Research Center has a good, very handy key at: https://stroudcenter.org/macros/key/

One from Pennsylvania Sea Grant has color photos and indicates pollution tolerance: https://seagrant.psu.edu/topics/pa-coastal-ecosystems-ecosystem-health-education-stream-restoration-and-stormwater-management.

When I collect macroinvertebrates on Middle Spring Creek, I usually see a few crawlies immediately. Saturday on the Conodoguinet, I saw nothing moving in my net, despite using it several times, but there were plenty of macros when I started observing the results in the plastic bin. I probably missed them in the net because most were much smaller than those I find at Middle Spring. Others were snails or bivalves that do not move around when you first scoop them up. Among the group, we found some of each of the EPT types, but relatively few stoneflies and caddisflies. We also found damselfly nymphs, water pennies, midges, crayfish, scuds, aquatic sowbugs, aquatic worms, as well as abundant snails, freshwater mussels and clams.

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