Diatoms vulgare is considered to be a current -loving winter 

 dominant that grows over a wide range of temperatures centered on 

 15° C (Lowe 1974) . Lowe (1996) lists Diatoma vulgare among lake 

 algae that tolerate high light, high turbulence, and broad 

 temperature fluctuations. Its abundance in Mol Heron Creek 

 probably reflects the cool, turbulent flow of this stream. 



The third co-dominant diatom in Mol Heron Creek was Cymhella 

 silesiaca (Table 4) . This species is a rheophilous (current- 

 loving) Summer and Fall form that is somewhat tolerant of organic 

 pollution (Lowe 1974, Lange-Bertalot 1979) . The fourth major 

 species was Rhoicosphenia curvata, a sensitive, attached diatom 

 and a common epiphyte on Cladophora. 



All but one of the diatom association metrics indicate that 

 Mol Heron Creek has excellent biological integrity for a mountain 

 stream (Table 4). A small percentage (0.97%) of teratological 

 diatoms indicates that Mol Heron Creek may suffer from chronic 

 heavy metals toxicity. However, abnormal diatom cells may result 

 from other factors. Most of the abnormal cells observed in Mol 

 Heron Creek were of Diatoma vulgare. This taxon belongs to the 

 family Fragilariaceae, which is the group of diatoms most likely 

 to produce abnormal cells. A relatively large percentage of 

 abnormal cells of Diatoma vulgare has been reported from the 

 Madison River in southwest Montana at a site that has good water 

 quality (unpublished data) . 



Although within acceptable limits for a mountain stream, the 

 pollution index for Mol Heron Creek was close to the threshold 

 indicating minor impairment from nutrient enrichment (Table 4) . 

 Taken together with the dominant diatom species and the small 

 percentage of Epithemiaceae, the relatively low pollution index 

 indicates a low to moderate level of organic loading and nutrient 

 enrichment in Mol Heron Creek. 



