COS 139-2 - Calibration and testing of stressor-specific genus sensitivity values and a genera sensitivity ratio metric to assess the causes of impairment in streams

Abstract: Effective water quality management is based on associations between at least two pieces of information: a stressor and a response. However, assessments are hindered by the lack of pre-developed stressor-response associations. To remedy this, we developed stressor-specific genus sensitivity values (SVs) for up to 707 genera to estimate sensitive genera ratios (SGRs) for as many as 34 common stream stressors. The SVs were estimated from a large, paired macroinvertebrate and environmental data set for the contiguous United States. Environmental variables measuring potential stressors were selected that were generally uncorrelated and usually had several thousand station observations. We calculated relative abundance weighted averages (WA) for each genus and environmental variable meeting data requirements in a calibration data set. Each environmental variable was split into ten intervals along each stressor gradient. Genera were assigned an SV from 1 to 10 based on the interval consistent with the WA for each environmental parameter. Using the calibration derived SVs, SGRs were calculated for the calibration and a validation subsets. SGRs are the number of genera with SV < 5 divided by the total number of genera. In general, as stress increased, the SGR (range: 0 – 1) significantly decreased for many environmental variables. At least disturbed stations, the SGRs were > 0.6 for 21 of 34 environmental variables and these 21 variables were assessed as having good discriminatory power. Regional performance of SGRs was evaluated by subdividing the calibration data set into West, Central, and East subsets and recalculating SVs. SGR mean absolute errors were smallest in the East and Central regions. Four case examples illustrate the use of SGRs to generate evidence of genera-specific alteration and evidence of sufficiency for identifying probable causes of stream impairment. These stressor-specific SVs expand the available tools for assessing stream biological impairments from commonly encountered environmental stressors.