Based on these affinities, the water at the confluence of the Biqhorn and the 

 Yellowstone might be described as nutrient enriched, circumneutral , alkaline, 

 moderately brackish, moderately hard, cool, well oxygenated, and flowing. The 

 relatively high diatom diversity indicates stability and health in the periphyton 

 community. 



Although not abundant, two additional taxa were found that are very char- 

 acteristic of brackish water : Biddulphia laevis (Fig. 3) and Mastogloia sp. 

 A single cell was found of Pinnularia subcapitata var. paucistriata (Fig. 4) , 

 a diatom preferential to fresh water of low mineral content. 



Discussion and Conclusions 



With one exception, the algae identified in net samples from the Yellowstone 

 and Bighorn Rivers in April 1973 were members of the periphyton or aufwuchs com- 

 munity. That exception, Fragilaria crotonensis , was significant only in the 

 Bighorn River sample where it ranked third in abundance. Wright and Soltero (6) 

 reported F. crotonensis as the most important organism of the phytoplankton in 

 Bighorn Lake on a cell volume basis and the dominant member of the early spring 

 diatom pulse. The colonies of F. crotonensis observed at the mouth of the Big- 

 horn River were probably produced in Bighorn Lake, discharged from the dam, and 

 carried downstream with the current of the river. In this reach the so-called 

 "phytoplankton" of the Yellowstone River therefore consists of sessile algae dis- 

 lodged from the bottom, giving the appearance of a plankton. Nevertheless, this 

 pseudo-plankton may be important downstream to filter feeding fish such as the 

 paddlefish. The findings of Williams (5) indicate that something approaching 

 a true river phytoplankton may develop in the lower reach of the river. 



The degenerate quality of the Cladophora at these stations leads one to sus- 

 pect that either (1) conditions are not suitable for growth in the immediate vicinity 



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