Macroinvertebrate Sample Collection

Methods used to collect aquatic macroinvertebrates are documented in the Vermont Department of Environmental Conservation Field Methods Manual (VTDEC 1989). All macroinvertebrate samples are collected during the late-Summer, early fall index period, from September to mid-October. A two-person field crew selects a representative section of the stream reach to be sampled. The preferred habitat for assessment is a "riffle" section of the stream. (A database on low gradient, sand and silt bottom streams in being developed.) Physical characteristics recorded at each site include: stream width; depth; water velocity; water temperature; specific conductance; weather conditions; substrate composition; substrate embeddedness (riffle sites only); canopy cover; stream bank condition and immediate upstream land use. All data are entered onto a field sheet (pdf, 40 KB) with appropriate site and sampling event identifiers, along with additional comments that may be pertinent to the site evaluation. A water sample is collected for pH, alkalinity, conductivity and color to help determine stream characterization and type.

Samples are collected using an 18 inch wide x 12 inch high D-frame net with a 500 u mesh size. The goal of sampling is to collect a sample that is representative of the overall biological integrity within a stream reach. The net is placed in the riffle at an appropriate location and an area immediately upstream of the net is thoroughly disturbed by hand, ensuring that all pieces of substrate are moved and rubbed clean of attached organisms. Moving up-stream, this is repeated at 4 different locations within the riffle, representing a range of velocity and substrate type characteristic of that riffle. Each specific location is actively sampled for about 30 seconds, and active sampling is terminated at the end of two minutes. Time spent relocating to a new area within the riffle is not counted as part of the two minutes. The contents of the net are washed into a quart mason jar and preserved with 75% ethanol. Two replicate samples are collected from the same reach, being careful to avoid areas previously disturbed. This "composite" sampling methodology effectively collects samples representative of the macroinvertebrate community of that riffle.

All methods used to process aquatic macroinvertebrate samples are documented in the Vermont Department of Environmental Conservation Field Methods Manual (VTDEC 1989). All sample processing is done in a laboratory setting. Processing includes picking organisms from the sample, sorting the organisms into taxonomic groups, identifying organisms to lowest possible taxonomic level, and entering data into the data management system.

An entire sample is thoroughly washed through a # 30-mesh brass sieve. The sample is then back-washed into a 12 x 18 inch white enamel tray that has been marked so as to delineate 24 numbered equal squares. The sample is spread evenly over the tray surface. A random number between 1 and 24 is selected and picking is started on that square in the tray. All organisms are removed from a square before proceeding to the next sequentially numbered square. Picking continues into subsequently numbered squares until a minimum of six squares (25 percent of the sample) have been picked. If less than 300 organisms have been picked at this point, picking continues until a total of 300 organisms have been removed or the entire sample has been picked, whichever comes first. Sub-sampling details are recorded on bench sheets. Removed organisms are sorted to order and placed in appropriately labeled vials in alcohol for further identification. If the sample has not been totally picked, the remaining sample is qualitatively examined for Ephemeroptera, Plecoptera, Trichoptera (EPT) taxa and other larger animals not found in the sub-sample. Organisms are removed, labeled, and stored separately from the sub-sampled organisms. These separated animals are used primarily to track species distribution.

All organisms are subsequently identified to the lowest practicable taxonomic level by staff specializing in a specific macroinvertebrate order. Identifications are recorded on laboratory bench sheets. The data management system normalizes all abundance data to a standard sampling effort to account for variations in sub-sampling procedures. The data management system uses "scripts" to calculate and report out the mean percent composition and density of all taxa, the standard error (based on the mean of two replicates) of all taxon abundance estimates, the functional group percent composition, and a wide range of community biometrics for each sampling event in a sample summary report. Taxa richness is manually adjusted for each sample to account for differing levels of taxonomic identification within a sample. The biometrics are electronically transferred to a macroinvertebrate metrics data table and the adjusted taxa richness values are inserted. From this table a site summary report is generated, which includes all sampling events from a site over time.

Additional Information

• Calculation of Metrics (pdf, 53 KB)

Updated: January 2012