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Benthic Macroinvertebrates

Benthic macroinvertebrates are the most widely used biological assemblage for monitoring in state water resource agencies. They are susceptible to degradation of water, sediment, and habitat, and therefore serve as good indicators of localized environmental conditions.

There are many advantages to using this assemblage. They are affected by various short-term environmental stressors throughout their different life stages. Certain stages will be more sensitive than others to particular stressors. Benthic macroinvertebrates in a sensitive life stage will respond quickly to stress, while the entire assemblage will respond more slowly. Benthic macroinvertebrates also serve as a primary food source for many fish, and healthy fish communities are of great concern to most state water resource agencies. Finally, benthic sampling is relatively easy and members of the assemblage are generally abundant in most waterbodies.

Technique 1 (see equipment list)
The single habitat approach to field sampling is the method that was emphasized in the original RBPs. Benthic macroinvertebrate diversity and abundance are usually greatest in cobble substrate or riffle/run habitats, so sampling usually focuses on these habitat types. An alternate habitat is sampled when the cobble substrate type represents less than 30% of the sampling area based on the reference streams for the area. Before sampling, a 100-meter reach (or some multiple of the stream reach) of the stream should be selected that is representative of the overall characteristics of the stream. The sampling crew should draw a map of the sampling reach and document local land use, site description, and weather conditions.

A composite sample is taken from individual spots in the riffles and runs that represent different velocities. Sampling starts at the downstream end of the reach and proceeds upstream. There are various types of sampling nets that can be used, but sampling is usually done with a 1 m kick net. The "kicks" collected from different locations in the stream reach will be composited for a single sample. The percentage of each habitat type in the reach is recorded on the field sheet. The sampling conditions and gear types used should be noted, along with a qualitative estimate of macroinvertebrate composition and abundance. After the sampling is completed, the samples are preserved and returned to the laboratory.

Technique 2 (see equipment list)
The multihabitat approach is preferred for streams dominated by sandy or silty sediments or with variable habitat structure. This method ensures that major habitat types are sampled in proportion to their occurrence in the sampling reach. A 100-meter reach of the stream should be selected that is representative of the overall characteristics of the stream. The samplers should then draw a map of the sampling reach and document local land use, a site description, and weather conditions. Sampling will begin at the downstream end of the reach and proceed upstream. The benthic macroinvertebrate samples are collected from each instream habitat by kicking the substrate or by jabbing it with a D-frame dip net. A total of 20 jabs (with the D-frame dip net) or kicks (with the kick net) will be taken from the entire sampling reach. Each habitat type should be sampled in proportion to its occurrence in the reach. The organisms collected are then composited into a single sample. The samples should be preserved in 95% ethanol and labeled.

Technique 3 (see equipment list)
The biological reconnaissance (BioRecon) is a biological survey technique that can be used to screen potential sampling sites and then differentiate between impaired and non-impaired sites. When performed by an experienced, locally knowledgeable biologist, a large number of sites can be rapidly screened, and the areas needing further study can be targeted. This approach is used to cut down time spent in the laboratory and with analysis. Either field or laboratory identification can be done with this technique.

A 100-meter reach that is representative of the stream is selected for the BioRecon. Site description, weather conditions, and land use are recorded on the field data form. Sampling begins at the downstream limit of the reach and proceeds upstream. Each major habitat type in the reach should be sampled for a total of four jabs or kicks over the entire reach. The collected organisms are composited into a sieve bucket to produce a single homogeneous sample. Sorting is done at the site, and organisms are identified and recorded on a tally sheet. A representative of each taxon is preserved and verified in the laboratory. Analysis is done by comparing the value of each metric to a predetermined value for the stream class. Sites with metric values below the threshold value are considered "suspect" of being impaired, and these sites may require further investigation and sampling.

Laboratory
Benthic macroinvertebrate samples should be processed in a laboratory under controlled conditions. Subsampling of benthic samples is sometimes used (although some scientists discourage it) to reduce the effort needed for sorting and identifying the macroinvertebrates, as keying out specimens can sometimes require long hours. To obtain a subsample, the entire composite sample should first be rinsed and then spread evenly across a pan marked with a grid. Using a random numbers table, four squares on the grid are selected, and the organisms within this grid are used as the subsample. If still too many organisms are collected, the subsample can be spread out again and the process can be repeated. The final subsample should be placed in glass vials and preserved with 70% ethanol.

Taxonomic identification of the organisms can be done to any level, but it must be done consistently among samples. Identification to genus or species level provides more specific information about sensitivity to impairment and on ecological/environmental relationships. Identification to the family level requires less expertise to perform and will usually speed up the assessment process.

Metrics for benthic macroinvertebrates have been used in several different indices. The metrics that are most effective are those that have differences in response across a range of human influences. (List of recommended metrics)

All three approaches to sampling the benthic macroinvertebrate assemblage are discussed in the protocol. Click on the box below to review the complete protocols now, or continue with the remainder of this module and visit the full document site later.

Link to Chapter 7 of RBPs document