USGS-NWQL: B-9135-00 (Qualitative): Benthic Macroinvertebrate Sample Processing: Qualitative Visual Sort 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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Samples are visually sorted under a light by a taxonomist for up to 2 hours. Samples are first size-fractionated to separate coarse and fine organic debris to increase sorting effectiveness. The coarse-size fraction is sorted for about 0.25 hour, while the fine-size fraction is sorted for up to 1.75 hours. Sorting is focused on mature, undamaged organisms that can produce Genus- or Species-level taxonomic resolution. Immature or damaged specimens are sorted if it is likely that they represent new taxa from the sample. The objective of sorting is to find as many distinct taxa as practical within the 2-hour limit. Studies performed in the BG indicated that the rate of accrual of new taxa diminishes substantially after 2 hours of visiual sorting; therefore, the visual sorting period used in the qualitative method is limited to 2 hours. |
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Scope and Application
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The goal of the qualitative processing method is to produce a comprehensive list of BMI taxa present in a sample. The abundance of each taxon is not determined. |
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Applicable Concentration Range
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Interferences
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Sorting effectiveness varies with the type and amount of sample detritus. An excessive amount of organic detritus reduces one's ability to adequately differentiate organisms (especially small, cryptic organisms) from the sample matrix. Large clumps of algal filaments must be carefully separated, and delicate organisms (for example, mayfly larvae) must be carefully handled to minimize damage or loss of taxonomically valuable body parts, such as gills and legs. Consequently, samples with large amounts of organic detritus or filamentous algae are difficult to sort and may have large numbers of damaged specimens. |
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Quality Control Requirements
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After at least 25 percent of the sample has been sorted, a second taxonomist scans the sorted remnant for obviously missed or under-represented taxa for about 0.25 hour to ensure that the sample is sufficiently sorted. This QC step is performed before the completion of sorting so that recommendations can be implemented, while the taxonomist sorts the remainder of the sample. |
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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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Less than 3 hrs. per site/sample |
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Sample Preparation Methods
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