Benthic monitoring in a stream involves monitoring macroinvertebrates. Macroinvertebrates are an extremely important part of river ecosystems.

Benthic means bottom-dwelling and refers to the organisms that live in, crawl upon, or attach themselves to the bottom (or substrate) of the river. Most benthic macroinvertebrates are aquatic insects or the aquatic stages of insects. They also include such things as clams and worms.

Collecting macroinvertebrates can provide an understanding of a river's condition. Because many macroinvertebrates live in the stream year-round and sometimes over multiple years, their presence or absence provides valuable information about a river's health over time.

The methods described here generally follow a qualitative or semi-quantitative approach. They have been used successfully by non-biologists to monitor water quality and identify trouble spots.

• Choose a 3-foot by 3-foot area in a riffle typical of the stream. A riffle good for sampling will have cobble-sized rocks, fast moving water, and a depth of 3 to 12 inches.
• Use the kick seine method to collect the sample. Enter the water downstream of your sampling location. Have one person hold the poles on the net upright in the sampling area and push the poles firmly into the substrate. No water should wash under or over the net. Place small rocks on the bottom edge, if needed, to hold it down.
• As the net is held, a second person brushes all organisms off the cobbles and rocks. Completely brush rocks and set them outside the sampling area. Next, stir the substrate to a depth of several inches to dislodge any burrowing macroinvertebrates. You can use a metal tool, such as a long screw driver, or use your feet in a shuffling motion to kick the substrate toward the net. Stir for at least one minute.
• Lift the kick seine out of the water with a forward scooping action to avoid losing macroinvertebrates off the end of the screen.
• After carrying the seine to the stream bank, wash the contents into a large bucket filled with stream water. Washing from behind the screen helps to dislodge the animals. Use only stream water for rinsing organisms off the net into the bucket. If you plan to sort back at your lab or school, you can transport the sample in the large bucket, preferable with a lid, with plenty of stream water.
• You can begin sorting directly from the bucket into ice cube trays or other sorting containers filled with stream water. However, it is often easier to scoop parts of the sample into shallow pans from which to sort. Keep scooping and sorting until you've identified the entire sample (or reached whatever maximum number of organisms your program prescribes). Sort the different taxa (types of specimens) into separate containers or sections of the ice trays.
• Once you have sorted the organisms, they can be identified in the field or lab using identification keys and guides.
• Record the results on the data sheets.

The organisms in Group I and Group II are considered “sensitive” and “somewhat sensitive” to poor water quality, so they cannot survive in streams that are highly contaminated. The organisms in Group III are relatively tolerant to poor water quality conditions. In general, if you find more macroinvertebrates from Group I, it is likely that your stream is fairly healthy. However, if you find more organisms from Group III and few from Group I, the stream probably has poor water quality.

Informational Links

• Benthic Key • Taxonomic Key to Benthic Macroinvertebrates
• Data Sheet • Streamside Biosurvey: Macroinvertebrates