Benthic macroinvertebrates (BMI) are a useful indicator of stream health. BMI are relatively quick, inexpensive, and can indicate the presence (or absence) of pollutants in a water body. They have predictable community composition under natural conditions and provide a snapshot of long term conditions. Some BMI are especially useful for targeted sampling due to their high sensitivity to environmental changes and pollutants. For quantification, models compare the taxa observed at the site to the taxa expected in the absence of human-caused stress.
A reach for BMI sampling should be long enough to present a representative picture of the streams macroinvertebrate community. A reach, typically 40 times the width of the stream, should be dominated by riffle habitat- as riffles host a higher diversity of macroinvertebrates. If riffle habitat is unavailable, the next best habitat is selected.
Benthic macroinvertebrate collection consists of subsamples from 8 targeted locations. The sampler moves through a reach of river collecting subsamples with a D-net using a 500μm sieve bucket to sort the macroinvertebrates from sediment and other debris. This technique is designed to be rapid so that each subsample takes no more than 3 minutes to collect. This, however, takes some practice to become familiar with the method and stream habitats. See the instructions and SOP in the Appendix for details.
For detailed instructions, see the Standard Operating Procedure (SOP) provided in your notebook, or follow the link at the bottom of the instructions.
For a pdf of the instructions, check out the Tier 2 Monitoring Manual
Riffle – Water that moves over a shallow area of cobbles and gravel creates a riffle (a length of stream characterized by shallow, fast moving water broken by rocks).
Pool – A deep area of fairly still water which creates refuges for fish to hide in and to rest from the current.
Glide – A shallow stream reach where the water is moving more slowly.
Reach – A section of stream used for assessment.
Run – An area where the water is flowing rapidly and is deeper than a riffle.
Rapid – Water movement is rapid and turbulent; surface with intermittent “white water” with breaking waves.
Establish a sampling reach and subsample locations:
- Establish average wetted width. Multiply this width by 40 to get the length of the reach to sample.
- Put a stake at the start and end of the reach.
- Take GPS points within the reach and record the coordinates, best done in the center of the reach.
- Complete a rough sketch map of the designated stream reach being sure to note any interesting features or landmarks/directions that can be used to find the reach in future visits along with the GPS points.
- The collected sample is made up of 8 sub samples within the reach targeting riffle
habitat. If no riffles are available, the edge may be targeted.
- To reduce human bias, alternate locations in the stream (e.g. Left- 25% of channel width, center- 50% of channel width, right- 75% of channel width). Start randomly with one of these locations and consistently follow the pattern of left (L), center (C), right (R) and repeat until all 8 subsamples are collected (See Figure 1 below). Multiple subsamples can be collected in one riffle habitat if riffle habitat in the reach is limited.
- Begin walking upstream along the reach. While walking, look for desirable habitat: riffle/runs with coarse substrates. Target course substrates such as gravel (pea-sized and larger) to small boulders (basketball-size and smaller). If course substrates are lacking, woody debris, macrophytes (submersed plants) or leaf packs could be targeted. Identify and document these factors on the field sheet.
At each collection point, determine:
- habitat type- pool, glide, riffle, or rapid
- substrate- fine/sand, gravel, course or other (other includes occurrence of wood, leaves, edge habitat overhanging vegetation, bedrock, hardpan, etc)
- At each of the collection points, follow specific instructions for each habitat type.
Figure 1. Sampling locations within a reach. (Image not to scale)
Collection a sample in riffle/run habitats:
- Make sure net opening is facing upstream, so flow is directed into the net and eliminating gaps under frame (avoid large rocks that prevent full stream bed contact)
- Collect all available BMI within the area upstream of the net opening (net width wide x net width
- Disturb substrate w/hand, be sure to pick by hand larger organisms like mussels and snails.
- Pick up loose rocks or other substrate and dislodge organisms washing them into the net, ensuring substrate remains in front of the net opening and flow directed into the net.
- After scrubbing larger substrates (> golf-ball size) and removed from the area in front of the net, focus on the smaller substrate to a depth of 3 inches. Vigorously perturb smaller substrate within the quadrat for 30 seconds with your hands. May use your boot if too deep
- Immerse the net in the stream several times to remove any remaining contents.
- Repeat this process at the seven remaining sites.
Preservation and transport of the sample:
- Transfer contents of net and sieve bucket into a 2.5 gallon bucket, inspecting the net/sieve for any remaining bugs that may still be clinging to it. Using a wash bottle full of stream water and/or forceps, flush/pick them off the net and into the bucket.
- Fill the bucket with stream water to cover the collected materials.
- Swirl the bucket to suspend lighter organic materials (sticks, leaves, organisms). Pour into sieve bucket leaving the heavier material (inorganic) in the 2.5 gallon bucket.
- Continue rinsing and swirling until no more bugs are seen crawling around in the 2.5 gallon bucket and are back in the sieve bucket. Be sure to inspect the bucket for caddisfly cases as sometimes the cases are composed of gravel-sized materials.
- Place the new contents of the sieve bucket into a 1L sample bottle by hand, no more than 40% full. Use multiple jars if necessary. Check to make sure no organisms are left in the bucket. If so, use a pair of forceps to pick the bugs out.
- Completely fill the jar with 95% ethanol (no head space).
- Slowly tip the jar to to a horizontal position and then gently rotate the jar to mix the preservative. Do not shake. After mixing, seal the jar lid with electrical tape. Place the sample label on the jar. Label includes Site name, STORET #, Initials, date, # of bottles (1 of 1) or (1 of 2) and equipment/method (D-net; 8 subsamples)
- Store filled jars in an empty cooler or jar tote during transportation. Samples do not need to be refrigerated or stored on ice.
After collection, follow decontamination procedures to reduce the spread of invasive species.
Aquatic Benthic macroinvertebrate Collection SOP: