a group, the diatoms were the most abundant and diverse. One genus of red and two 

 genera of bluegreen algae were also present. 



No less than 75 varieties and 23 genera of diatoms were identified from the 

 acid cleaned material (see Table II). The diatom species composition of the three 

 samples was similar, differing primarily in relative abundance of individual taxa. 

 Nitzschia dissipata (Fig. 1) was the most common diatom species, followed in order 

 by Cocconeis placentula var. euglypta , Gomphonema olivaceum , and Achnanthes 

 minutissima . 



Of the diatoms found at the three stations, 29 taxa are sessile, growing 

 attached to the bottom or to other plants. Forty-five taxa are either mobile 

 over some substrate or quasi-planktonic and forming short filaments resting upon 

 but not attached to a substrate. Only one species, Fragilaria crotonensis (Fig. 2), 

 is truly planktonic and capable of livinq indefinitely suspended in open water. 

 F. crotonensis was common only in the Bighorn River sample where it ranked third 

 in volume (Table I). Of the remaining 10 genera identified in the raw samples, 

 four are sessile and attached and six are quasi-planktonic. 



The ecological requirements of each of the more common diatom species were 



determined according to Patrick and Reimer (1). A summation of these requirements 



and the number of times they were repeated is given below: 



nutrient enriched (6) 



circumneutral (5) 



oligohalobe (4) 



salt indifferent (4) 



alkaline (4) 



fresh to slightly brackish (4) 



eurytopic (3) 



cool (3) 



moderately hard (2) 



flowing (2) 



high oxygen (2) 



alkaliphil (1) 



highly calcareous (1 ) 



brackish (1) 



moderate conductivity (1) 



