EFFECTS OF POWER-PLANT OPERATION 607 



There were some changes in the pattern of class dominance over 

 the 4 years, especially for the Chlorophyceae by density, but there 

 were no obvious changes in the patterns of fluctuation in density or 

 biovolume abundances by the classes after power-plant operation 

 commenced. There was, however, somewhat less variability in the 

 distribution of class abundances among the stations in the postopera- 

 tional years when power-plant circulation connected the intake, 

 discharge, and main-lake sections of the impoundment. The most 

 notable example was the marked abundance of Fragilaria crotonensis 

 Kitton, which coincided with the first month of pump operation 

 (June 1974, one month before steam generation commenced) and 

 resulted in Bacillariophyceae biovolume peaks at every station on the 

 lake. Two other patterns that appear to have coincided with the 

 postoperationsil years were the development of low but consistent 

 abundances of the Cyanophyceae and the decrease in occurrence of 

 notable abundances of Chrysophyceae. 



Phytoplankton Taxa 



The Chlorophyceae contained 15 taxa that attained dominance 

 in one or more samples by density and 8 by biovolume. The 

 Dinophyceae were second in importance by biovolume, with 7 taxa, 

 and the Bacillariophyceae contained 6 dominant taxa by density and 

 biovolume. In this study the principal dominant taxa were con- 

 sidered to be those exhibiting a notable frequency of dominance and 

 subdominance by either density or biovolume. These taxa are listed 

 in Table 2. 



Succession in the principal taxa contributing to the yearly 

 phytoplankton cycle has occurred over the 4 years, as Table 2 and 

 Fig. 2 further illustrate. The tiny colonial blue-green alga 

 Agmenellum quadriduplicatiim (Menegh.) Breb. was a principal 

 summer dominant by density in years 3 and 4 but made no further 

 notable appearances. In these same 2 years, the small filamentous 

 centric diatom Melosira italica var. alpigena (Grun.) A. Cleve was a 

 principal autumn and winter dominant. In year 5 its importance was 

 greatly decreased, however, and by year 6 it had become a minor 

 taxon. The major importance of Selenastrum minutum (Naeg.) 

 Collins, a small lunate-celled green alga, was in the autumn and 

 winter of year 3. Although it was also important in autumn and 

 winter of the following 2 years, by year 6 it too was no longer 

 dominant. The minute spherical-celled green alga, Nannochloris sp., 

 on the other hand, went from a dominant in only a few spring 

 samples in year 3 to become the most important principal taxon of 

 summer and winter in year 4. It continued this distribution of 



