INTRODUCTION 



Aquatic invertebrates are aptly applied to bioassessment since they are known to 

 be important indicators of stream ecosystem health (Hynes 1970). Long lives, complex 

 life cycles and limited mobility mean that there is ample time for the benthic community 

 to respond to cumulative effects of environmental perturbations 



This report summarizes data collected in September 2000 from two sites on the 

 Shields River, Montana A multimetric approach to bioassessment was applied to the 

 data: this approach uses attributes of the benthic invertebrate assemblage in an integrated 

 way to measure biotic health. A stream with good biotic health has been described as 

 . . .a balanced, integrated, adaptive system having the full range of elements and processes 

 that are expected in the region's natural environment. . ." (Karr and Chu 1999). 



The additive muhimetric approach designed by Plafkin et al. (1989) and adapted 

 for use in the State of Montana is ". . . an artay of measures or metrics that individually 

 provide information on diverse biological attributes, and when integrated, provide an 

 overall indication of biological condition." (Barbour et al. 1995). Community attributes 

 that can contribute meaningfully to interpretation of benthic data include assemblage 

 structure, sensitivity of community members to stress or pollution, and functional traits. 

 Each metric component contributes an independent measure of the biotic integrity of a 

 stream site, combining the components into a total score reduces variance and increases 

 precision of the assessment (Fore et al. 1994) Effectiveness of the integrated metrics 

 depends on the applicability of the underlying model, which rests on a foundation of 

 three essential elements (BoUman 1998). The first of these is an appropriate stratification 

 or classification of stream sites, typically, by ecoregion. Second, metrics must be selected 

 based upon their ability to accurately express biological condition Third, an adequate 

 assessment of habitat conditions at each site to be studied is advantageous to the 

 interpretation of metric outcomes. 



Implicit in the multimetric method and its associated habitat assessment is an 

 assumption of correlative relationships between habitat parameters and the biotic metrics, 

 in the absence of water quality impairment These relationships may vary regionally, 

 requiring an examination of habitat assessment elements and biotic metrics and a test of 

 the presumed relationship between them This writer (1998) has recently studied the 

 assemblages of the Montana Valley and Foothill Prairies ecoregion, and has 

 recommended a battery of metrics specific to that ecoregion, which has been shown to be 

 sensitive to impairment, related to habitat assessment parameters and consistent over 

 replicated samples. 



Habitat assessment enhances the interpretation of biological data (Barbour and 

 Stribling 1991), because there is generally a direct response of the biological community 

 to habitat degradation in the absence of water quality impairment. If biotic health appears 

 more damaged than the habitat quality would predict, water pollution by metals, other 

 toxicants, high water temperatures, or high levels of organic and/or nutrient pollution 

 might be suspected. On the other hand, an "artificial" elevation of biotic condition in the 

 presence of habitat degradation may be due to the paradoxical effect of mild nutrient or 

 organic enrichment in an oligotrophic setting. 



