2.55, and 12% of the statewide collections demonstrated taxa numbers and 

 diversities below 30 and 3.20 units during the warm weather season. 



In terms of interpretation as outlined by Ingman et al (1979) and Weber 

 (1973), stream periphyton samples with diatom species numbers and Shannon- 

 Wiener diversities around or in excess of the statewide means (i.e., greater 

 than about 40 taxa to a maximum near 67 with a diversity greater than 4.00 to 

 a maximum of about 5.00 units) would tend to be indicative of an excellent 

 biological health with the absence of any marked pollutive stress or other 

 perturbations. In general, periphyton collections with somewhat lower taxa 

 numbers between 25 and 40 and with somewhat lower diversities between 3.00 and 

 4.00 units are also indicative of fairly good environmental conditions. How- 

 ever, values in these latter ranges could be suggestive of the occurrence of 

 comparatively mild instream problems, and the likelihood and severity of such 

 a potential stress would be expected to be enhanced to some small degree as 

 the taxa numbers and diversities fall to the 25 and 3.00 level respectively. 



But as a more consistent and accurate reference guideline, periphytic 

 taxa numbers and diversities that lie below the 25 and 3.00 levels respective- 

 ly have been found to be more definitely suggestive of a pollutive problem. 

 Furthermore, a progressively greater severity of instream stress might be 

 anticipated with the lower diversity values in those instances where diver- 

 sities are found to reside in the 3.00 to zero range. Periphytic diversities 

 below 2.00, in turn, are particularly demonstrative of an extreme perturbation 

 with the zero value representative of the diagnostic limit. 



Periphytic evidence of somewhat marked environmental difficulties has 

 been uncovered for a small number of Montana's streams as revealed by the 

 minimum statewide taxa number and diversity readings that were listed 

 previously. However, the below 2.00 diversity extreme was not uncovered while 



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