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Biological Condition Index Development for the Truckee River: Benthic Macroinvertebrate Assemblage

Prepared By

Tetra Tech, Inc.
10045 Red Run Blvd, 110
Owings Mills, MD 21117

Prepared For

Nevada Division of Environmental Protection
July 2004


The objective of our analysis was to develop a biological index for benthic macroinvertebrates for the Nevada portion of the mainstem of the Truckee River using preexisting data. We analyzed data collected by four separate entities, Nevada Division of Environmental Protection (NDEP), Pyramid Lake Paiute Tribe (PLPT), Truckee Meadows Water Reclamation Facility (TMWRF), and Desert Research Institute (DRI). To develop a common dataset from collections of multiple entities, an evaluation of the field and/or lab methodological differences was performed to minimize non-random error. A common index period was designated as low flow (July to October), all replicates were combined, and the number of individuals per sample was set to 500 organisms (samples with more organisms were randomly sampled with a computer to 500).

A multimetric index was developed from the data set and comprised six metrics or attributes of the benthic macroinvertebrate assemblage; total taxa, percent Ephemeroptera, percent Chironomidae, percent dominant taxon, percent filterers, and percent clingers. In an assessment, each metric value is converted to a standard score ranging between 0 and 100 (100 being closest to reference or optimal value). The scores for all six metrics are averaged to obtain an aggregated index score and compared to thresholds or break points based on percentages of the overall score and expected conditions for a good quality benthic macroinvertebrate community. Application of the benthic index to the mainstem of the Truckee River indicated the biological condition was of higher quality in the upper reaches and declined in quality as the river approaches Pyramid Lake. A more definitive assessment is pending the results of the analyses on companion assemblages of the aquatic community, namely the fish and algal assemblages.

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