USGS-NAWQA: BS0333: Benthic Macroinvertebrate: Quantitative Fixed-Count Method for Processing Samples
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Official Method Name
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Quantitative Fixed-Count Method for Processing Benthic Macroinvertebrate Samples |
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Current Revision
| 2011 |
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Media
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Biological (Waterbody type - Wadeable stream) |
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Instrumentation
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Standard metal sieve |
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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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Protocol
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USGS NAWQA 2002 Eco Protocols - Revised Protocols for Sampling Algal, Invertebrate, and Fish Communities as Part of the National Water-Quality Assessment Program |
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Brief Method Summary
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The goal of this method is to standardize procedures used for the taxonomic identification of benthic macroinvertebrates (BMIs). These procedures can be applied to any BMI sample submitted to the National Water Quality Laboratory (NWQL) for analysis. Taxonomic identification of BMIs requires experienced personnel trained in zoological taxonomic principles and possessing a broad knowledge of all aquatic macroinvertebrate groups. Refer to the method document for steps on how to count the sample. |
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Scope and Application
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The goal of the quantitative fixed-count processing method (Appendix 1) is to sort, identify, enumerate, and estimate total relative abundance of benthic macroinvertebrate (BMI) taxa from a sample. The fixed-count method can be applied to BMI samples collected in the field using quantitative, semiquantitative, or qualitative sampling methods. The quantitative fixed-count method is based on a minimum number of organisms sorted from the sample, and is defined by a project’s data-quality objectives (for example, 100-, 200-, or 300-organism fixed-count target). Samples with more organisms than the fixed-count target are subsampled using a subsampling frame partitioned into 5.1 cm x 5.1 cm grids (Moulton and others, 2000). Multiple, randomly selected portions of the original sample (stage-1 grids) are selected and the average number of organisms in each stage-1 grid is determined. Using the average number of organisms in each stage-1 grid, an appropriate subsampling strategy is determined. Grids are randomly selected and sorted. Depending on the fixed-count target, total sorting time using this method is limited to a maximum of 8 hours. Large-rare organisms are sorted from any remaining unsorted portions of the sample. Organisms are identified and enumerated. |
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Applicable Concentration Range
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Interferences
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1.5.1. Sorting effectiveness varies with the type and amount of sample detritus. Excessive organic detritus makes it difficult to distinguish organisms (especially small, cryptic organisms) from the sample matrix. 1.5.2. Sample volumes in excess of 750 mL are difficult to process with this method because it is difficult to achieve a thin even distribution of sample in the subsampling frame; the volume must be reduced to less than 750 mL before using the fixed-count method. 1.5.3. Substantial amounts of inorganic material (for example, sand) or large organic debris (for example, sticks and leaves) inhibit the uniform distribution of a sample in a subsampling frame. Such debris should be removed before proceeding with the method (see SOP BS0331.0). 1.5.4. Substantial amounts of filamentous algae should be uniformly distributed in the subsampling frame as best as possible. Clumps of algal filaments must be separated carefully, and delicate organisms (for example, mayfly larvae) must be handled gently to minimize damage or loss of taxonomically valuable body parts such as gills and legs. |
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Quality Control Requirements
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8.1. Inspect all sieves, subsampling frames, and estimation trays before and after processing. 8.2. Verify that information recorded on the Fixed-Count Processing—Subsampling and Preliminary Enumeration Worksheet is correct prior to the large-rare sort. 8.3. Determine Sorting Effectiveness 8.4. Corrective Actions |
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Sample Handling
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Refer to NWQL SOP No. BS0331.0. for sample preparation. |
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Maximum Holding Time
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Each sample received in formalin should be washed and re-preserved in 70-percent ethanol within 2 weeks of receipt at the NWQL. Samples preserved in ethanol can be stored indefinitely. |
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Relative Cost
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Unknown |
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Sample Preparation Methods
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Each sample received in formalin should be washed and re-preserved in 70-percent ethanol within 2 weeks of receipt at the NWQL. Samples preserved in ethanol can be stored indefinitely. Ssee method documentation PDF for additional details. |
