USGS-NWQL: B-9135-00 (Quantitative): Benthic Macroinvertebrate Sample Processing: Quantitative Fixed-Count Method
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Official Method Name
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Methods of Analysis by the U.S. Geological Survey National Water Quality Laboratory - Processing, Taxonomy, and Quality Control of Benthic Macroinvertebrate Samples |
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Current Revision
| 2000 |
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Media
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WATER (Waterbody type - Wadeable stream) |
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Instrumentation
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Visual Comparison |
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Method Subcategory
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Population/Community |
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Method Source
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Citation
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Moulton II, S.R., Carter, J.L., Grotheer, S.A., Cuffney, T.F., and Short, T.M., 2000, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory-- Processing, taxonomy, and quality control of benthic macroinvertebrate samples: U.S. Geological Survey Open-File Report 00-212. |
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Brief Method Summary
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The principle objective of the fixed-count method is to identify and estimate the abundance of each Benthic Macroinvertebrate taxon sorted from the sample. This method is similar to the USEPA's RBP sample-processing procedure. The quantitative processing method is based on a fixed-count approach that targets some minimum count, such as 100 or 300 organisms. Organisms are sorted from randomly selected 5.1- by 5.1-centimeter parts of a gridded subsampling frame. The sorted remnant from each sample is resorted by a second individual for at least 10 percent of the original sort time. A large-rare organism search is performed on the unsorted remnant to sort BMI taxa that were not likely represented in the sorted grids. |
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Scope and Application
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Benthic macroinvertebrates (BMIs) are animals that live on or in the substrates (for example, sediments, woody debris, macrophytes, algae) of aquatic habitats such as lakes and streams. Typical examples of BMIs are flatworms, snails and clams, segmented worms, crustaceans, and aquatic insects. BMIs are used more frequently in water-quality studies than any other group of organisms. BMI data are frequently used to develop biocriteria that are used to rank aquatic systems according to their biological health. When combined with measurements of water chemistry and habitat, BMI data provide an integrated assessment of water quality in lakes and streams. The U.S.Geological Survey's National Water Quality Laboratory Biological Group (BG) processes BMI samples that have been collected by using a variety of techniques from diverse aquatic habitats throughout the United States. The BG has developed well-defined qualitative and quantitative processing methods that are sufficiently flexible to satisfy most data-analytic methods currently (2000) used for including estimates of BMI community composition in water-quality studies. The fixed-count method is normally used to process BMI samples that have been collected using a quantitative or semi-quantitative sampling method. However, the fixed-count method can also be used to produce estimates of relative abundance of the taxa sorted from qualitatively collected samples. |
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Applicable Concentration Range
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Interferences
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Inorganic debris in the sample matrix interferes with the uniform distribution of the sample matrix in the subsampling frame. Substantial amounts of inorganic debris are separated from the sample matrix by elutriation before distributing the organic portion of the sample in the subsampling frame. Large organic detritus is removed, rinsed, inspected for attached organisms, and then discarded. Samples that contain substantial amounts of filamentous algae are distributed as evenly as possible. The algae are cut by using scissors to aid in removing randomly selected grids from matrices that contain filamentous algae. A large sample matrix also inhibits efficient subsampling and sorting. The total volume of most samples collected from about 1 square meter can be sufficiently field processed to reduce the submitted volume of the sample to <750 mL. Laboratory splitting is sometimes necessary if the total submitted sample volume exceeds 750 mL. |
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Quality Control Requirements
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Quality control procedures are listed in the method for (1) Sorting effectiveness, (2) Documentation, and (3) Verification of taxonomic identifications. |
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Sample Handling
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BMI samples can be processed onsite to create several different sample components (main-body, large-rare, elutriate, and split). The extent of this process depends not only on decisions made onsite at the time of sample collection but on the subsequent laboratory processing methods desired. A brief description of each of these components is presented in Appendix 1. |
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Maximum Holding Time
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Within 2 weeks of receiving a sample, the original field preservative (typically 5 - 10 percent buffered formalin) is decanted through a sieve in a fume hood. The sample is then rinsed with water and preserved with 70-percent ethanol until processed. |
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Relative Cost
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3 - 6 hrs. per site/sample |
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Sample Preparation Methods
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