This genus is generally the commonest of desmids in the freshwater 

 plankton, and several species are strictly planktonic in habit (10). 

 Besides Staurastrum , of the 19 most common taxa in the Yellowstone, 

 Hynes (4) mentions Pediastrum , Anabaena , Scenedesm us, Ankistrodesmus 

 and Lyngbya as possible members of the potamoplankton. The most 

 common of these in the Yellowstone was Scenedesmus , which occurred 

 in 15.4 percent of all samples but, curiously, was more than twice 

 as common in the periphyton than in the plankton (Table 1). This 

 is understandable considering Hynes' explanation that many plankters 

 are of benthic origin or are derived from backwaters and side channels, 

 None of these planktonic non-diatom algae ever dominated a Yellow- 

 stone sample, which is to be expected when one considers that diatoms 

 almost always dominate the plankton of larger rivers worldwide (4), 

 as they apparently do in the Yellowstone River (Table 2). 



Several taxa exhibited seasonal preferences in the Yellowstone, 

 most likely in response to changing light and temperature regimes, 

 Enteromorpha , Closterium , Cosmarium and the planktonic Chlorophyceae 

 mentioned above clearly preferred the wanner and brighter months. 

 This agrees with Hynes' observation that green algae plankters gener- 

 ally occur only in summer or in permanently warm waters (4). Tem- 

 perature is also known to encourage development of blue-green algae, 

 but none of the Cyanophyta in the Yellowstone appeared to be clearly 

 favored by a rise in temperature concomitant with a change in season. 

 Hydrurus foetidu s and Ulothrix (including U. zonata) clearly preferred 



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