Using appropriate tools, microalgae were scraped, brushed, and/or 

 sucked from natural substrates in proportion to the rank of those 

 substrates at the study site. Macroalgae were picked by hand in 

 proportion to their abundance at the site. All collections of 

 microalgae and macroalgae were pooled into a common container and 

 preserved with Lugol ' s solution (APHA 1998) . 



Samples were examined to estimate the relative abundance and 

 rank by biovolume of diatoms and genera of soft (non-diatom) 

 algae according to the method described in Bahls (1993) . Soft 

 algae were identified using Dillard (1999) , Prescott (1978) , 

 Smith (1950) , and Whitford and Schumacher (1984) . These books 

 also served as references on the ecology of the soft algae, along 

 with Palmer (1977) . 



After the identification of soft algae, raw periphyton 

 samples were cleaned of organic matter using sulfuric acid, and 

 permanent diatom slides were prepared using Naphrax, a high 

 refractive index mounting medium, following Standard Methods for 

 the Examination of Water and Wastewater (APHA 1998) . For each 

 slide, between 401 and 412 diatom cells (802 to 824 valves) were 

 counted at random and identified to species. The following were 

 used as the main taxonomic and autecological references for the 

 diatoms: Krammer and Lange-Bertalot 1986, 1988, 1991a, 1991b; 

 Patrick and Reimer 1966, 1975. 



Lowe (1974) was also used as an ecological reference for the 

 diatoms. Bahls et al . (1984) provide autecological information 

 on important diatom species that live in the Fort Union Region of 

 Montana, including many of the diatom species found in Big Dry 

 Creek and Little Dry Creek. 



The diatom proportional counts were used to generate an 

 array of diatom association metrics (Table 2) . A metric is a 

 characteristic of the biota that changes in some predictable way 



