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

Summary
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.