environmental perturbations since a large coterie of miscellaneous species is 

 almost always typical of a healthy ecological system. At the same time, the 

 occurrence of a small selection of abundant forms is also descriptive of most 

 periphyton communities. In keeping with this theme, fifteen of the German 

 Gulch diatom taxa with mean PRA values in excess of 2.0% can be classified as 

 being conspicuous and common periphytic representatives of the project water- 

 way by demonstrating high abundances at one or more of the sites. 



The more common of the German Gulch diatoms can be listed as follows in 

 the order of their relative abundance levels and their mean PRA values: 

 Fragilaria vaucheriae (11.1%), Gomphonema olivaceum (9.1%), Cocconeis 

 placentula (9.1%), Achnanthes lanceolata (9.0%), Nitzschia dissipata (6.9%), 

 Navicula cryptocephala variety veneta (5.4%) , Hannaea arcus (3.8%) , Fragilaria 

 pinnata (3.1%), Rhoicosphenia curvata (3.1%), Achnanthes minutissima (2.8%), 

 Synedra ulna (2.5%), Nitzscia kutzingiana (2.5%), Cymbella affinis (2.3%), 

 Diatoma hiemale variety mesodon (2. 1%) , and Navicula tripunctata (2. 1%) . 



Ten examples of the 67 less common German Gulch diatoms with some 

 recorded in trace (t) amounts can be listed as follows: Amphipleura pellucida 

 (0.07%), Cymbella sinuata (1.3%), Didymosphenia geminata (t) , Eunotia perpu- 

 silla (0.17%), Meridian circulare (0.7%), Navicula lanceolata (t) , Nitzschia 

 palea (1.5%), Pinnularia borealis (0.1%), Stauroneis smithii (t) , and Suri - 

 rella ovata (1.0%). Furthermore, extremely large numbers of diatom species 

 and varieties (and genera) were not observed in the German Gulch samples 

 (e.g., Biddulphia laevis , Epithemia sorex , and Gyrosigma acuminatum ) . 

 However, such broad-ranging absences can be judged as commonplace through all 

 of the earth's biological assemblages, and the occurrence of missing taxa 

 thereby is certainly not unique to the German Gulch periphyton communities. 



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