Each sample was vigorously agitated, a few drops pipetted onto a glass 

 microscope slide, and portions of any large filamentous algae not taken 

 up in the pipet added to the subsample on the slide roughly in propor- 

 tion to their abundance in the sample. The slide therefore contained a 

 miniature of the orginal sample with both micro- and macroscopic algae 

 present in approximately the same proportion as in the original. 



The subsample on the slide was then covered with a coverslip and 

 this "wet mount" observed under low (lOOX) and medium (400X)magnifi ca- 

 tions. In scanning the subsample, all non-diatom taxa were identified 

 to genus. Where either diatoms or a non-diatom taxon clearly dominated 

 the sample, this was noted, along with the presence of non-algal micro- 

 organisms. 



Combining all stations on the Yellowstone, the percent frequency of 

 occurrence and percent frequency of dominance were computed for each 

 taxon, separately for the periphyton and plankton communities and col- 

 lectively for all samples regardless of community origin. Percent fre- 

 quency of occurrence was also computed for the 19 most common non-diatom 

 taxa at the 19 principal Department of Fish and Game stations on the 

 Yellowstone, thereby describing the longitudinal distribution of these 

 taxa. 



