Wednesday, June 6, 2007 - 11:45 AM
231

Precision of Field Sampling and Site Assessments Associated with Benthic Macroinvertebrate Indicators of Stream Condition in Montana

Benjamin K. Jessup1, James B. Stribling1, and David L. Feldman2. (1) Center for Ecological Sciences, Tetra Tech, Inc., 15 State Street, Suite 301, Montpelier, VT 05602, (2) Montana Department of Environmental Quality, 1520 East Sixth Avenue, Helena, MT 59620-0901

The Montana Department of Environmental Quality (MDEQ) uses two benthic macroinvertebrate indicators for detection of stream impairment, a multimetric index (MMI) and a predictive model of observed to expected taxa (O/E), each of which are calibrated to streams across the state.  As part of the routine monitoring program, a portion of the samples is replicated by spatial and temporal proximity.  Repeated sampling allows calculation of precision estimates, which are important for describing the uncertainty (systematic error) associated with field sampling and site assessments.  We evaluated 131 and 77 sample pairs for the MMI and O/E, respectively, using four different measures of precision, coefficient of variability (CV), 90 percent confidence intervals (90CI), relative percent difference (RPD), and percent difference for the final assessments.  Similar precision is reflected in the MMI and O/E; segregating the dataset and calculations by region and/or field method yields generally similar precision numbers for the indicators, although slightly better in the Mountains using the Hess sampler.  Assessments differed (impaired versus nonimpaired) in 18.3 percent of sample pairs for the MMI, and 19.5 percent for the O/E.  For both indicators, recommended measurement quality objectives (MQO) for CV is 10-15 percent and for RPD is 15-20 percent.