EFFECTS OF POWER-PLANT OPERATION 609 



seasonal importance in year 5 and was joined by Nannochloris 

 bacillahs Naumann in the winter of 1975. In year 6 it became the 

 most important taxon in all four seasons. The only other taxon to 

 approach this status as a perennial dominant was Chlorella sp., 

 another small-celled green alga, which in year 5 was a principal 

 dominant of all seasons except winter and in year 6 was the most 

 frequent subdominant. It was also important in the summer and 

 spring of year 3 but in year 4 was only a minor dominant. 



Of the six principal taxa by density in years 5 and 6, only one, 

 the needle-shaped pennate diatom Synedra filiformis Grun., was not 

 also a principal taxon in the first 2 years. Of the ten principal taxa of 

 the two base-line years, however, half did not occur again as principal 

 taxa after the power plant began operation. Taxa with moderate 

 frequencies of dominance by density were dominant or subdominant 

 in all sections of the lake each year in which they were present. The 

 few cases of localization exhibited in distribution of dominance were 

 limited to the period before power-plant operation began. 



By biovolume, only one principal taxon was common to more 

 than two consecutive years. This was Melosira italica var. alpigena, 

 which exhibited the same autumn and winter distribution of 

 importance in years 3 and 4 and to a lesser degree in year 5 but 

 became of minor importance in year 6. The dendritic colonial 

 chrysophyte, Dinohryon divergens Imhof., was the only other 

 principal taxon common to both years 3 (dominance in spring) and 4 

 (dominance in winter and spring). In the postoperational years, there 

 were only occasional biovolume dominances of this taxon. 

 Peridinium gatunense Nygaard and P. uolzii Lemm., two moderately 

 large armored dinoflagellates, were principal taxa common to years 4 

 and 5, with P. gatunense being the primary dominant in summer in 

 year 4, and P. uolzii an important dominant in the spring of year 4 

 and in both winter and spring of year 5. Neither taxon developed 

 notable biovolumes in year 6, however. Years 5 and 6 shared two 

 principal taxa, P. wisconsinense Eddy, another moderately large 

 dinoflagellate that was most important as a spring dominant in 

 year 5, and the smaller P. inconspiciium Lemm., which was the most 

 frequent summer and spring dominant of year 5 and became the 

 most important taxon of all four seasons in year 6. Thus, by the 

 fourth year of this study, P. inconspicuum had become the perennial 

 dominant by biovolume, just as Nannochloris sp. had become by 

 density. 



Three of the seven principal taxa in the base-line period were 

 dinoflagellates, and five of the nine postoperational-period taxa were 

 in this class, specifically in the armored genus Peridinium. The shift 



