606 CAMPBELL 



and the control station, 1118, were about equal in their distribution 

 of greater densities. 



Similar adjacent-station comparisons were made for total biovol- 

 ume abundances and total numbers of taxa per sample at the five 

 pairs of stations. The change in biovolume with power-plant 

 operation was similar to and even more marked than that in total 

 density. Biovolume differences of less than 300 mm-^ /m^ occurred in 

 only 48% of the adjacent-station samples in the nonstressed periods, 

 but, when the power plant began operation, 78% fell below this level. 

 Similar comparisons of the total numbers of taxa per sample, 

 however, showed no noticeable changes with power-plant operation. 



Phytoplankton Classes 



All the 11 algal classes that contain phytoplankton representa- 

 tives [according to the classification of Christensen, as modified by 

 Hommersand (1972)] were found in Belews Lake. By far the most 

 important class in each of the 4 years by density was the Chloro- 

 phyceae (the green algae). A number of classes attained important 

 dominance frequencies by biovolume. Most prominent of these was 

 the Dinophyceae (the dinoflagellates), followed by the Bacillario- 

 phyceae (the diatoms), the Chlorophyceae, and the Chrysophyceae 

 (the golden-brown algae). The frequency of dominance of the 

 Cyanophyceae (blue-green algae) was extremely low by both density 

 and biovolume determinations. 



The Chlorophyceae was by far the most frequent dominant by 

 density during all seasons of all 4 years; it was dominant in 76 to 

 78% of all samples in the base-line years, increasing to 100% in the 

 postoperational years. The Dinophyceae was the most frequent 

 summer dominant by biovolume of years 4 to 6. In years 5 and G it 

 was also the most frequent winter and spring dominant, and by 

 year 6 it had become the most frequent dominant in autumn. The 

 Bacillariophyceae, with dominance in 16 to 22% of the samples in 

 the preoperational years, was the most frequent dominant in the 

 autumn and winter of years 3 and 4 and in the spring of year 4. 

 During the 2 years of power-plmit operation, frequencies of diatom 

 dominance in these seasons became secondary to the dinoflagellates. 

 During the 4 years of study, the Dinophyceae steadily increased in 

 frequency of dominance, from 18 to 34 to 61 to 72%, respectively. 

 The classes with the greatest frequencies of dominance all tended to 

 exhibit similar degrees of importance throughout the lake. The few 

 examples of regional variability in dominance distribution by density 

 were found only in the preoperational years. 



