and the filaments observed originated far upstream or (2) early spring is normally 

 a period of scenescence for this taxon. Although the latter may be true, Cladophora 

 requires a firm, rocky substrate which may not be available in this stretch of 

 river. Diatoms are more adaptable and may colonize a rock, sand, silt or mud 

 bottom. In the upper reaches of the Yellowstone River, Stadnyk (3) reported 

 that "diatoms made up the vast majority of the autotrophic fauna (sic) at all 

 stations." He also observed a significant increase in siltation at his lowest 

 station (Laurel). Below this point conditions for growth of sessile green algae 

 may be less than optimum with the diatom flora achieving additional dominance. 

 Westinghouse (4) found no filamentous green algae at the mouth of Armells Creek; 

 the flora was almost completely dominated by diatoms. 



The flora of the Yellowstone at its intersection with the Bighorn is inter- 

 mediate between that of a high mountain stream and a lowland plains river. It 

 probably coincides quite closely with the transition zone between a cold water 

 and a warm water fishery. The Yellowstone does not appear to be influenced ap- 

 preciably by the introduction of the Bighorn, at least in terms of the algal 

 flora on the date these samples were collected. 



The relative abundance of algae reported herein may not be truly representative 

 of their abundance in the periphyton community since they were not collected from 

 their natural growth habit. Additionally, net samples may be selective toward 

 the larger forms since the extremely small species may pass through the meshes 

 of the net. 



Recommendations 



1. Future periphyton collections should be made without the use of nets, which 

 are selective for larger species. (For proper procedure, see "SUGGESTED 

 SAMPLING SCHEME", L. L. Bahls, April 15, 1974.) A fine-meshed plankton tow 



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