EFFECTS OF POWER-PLANT OPERATION 613 



the discharge station than at the intake station in the stressed period. 

 This may have been caused by temperature effects on the phyto- 

 plankton since this trend is consistent with results from other studies 

 of phytoplankton exposed to elevated temperatures. It was found 

 that a heat shock of limited duration within certain temperature- 

 tolerance limits of the algae can create incipient cell division and, 

 thus, result in larger densities (Foerster, Trainor, and Buck, 1974; 

 Goldman and Carpenter, 1974; Patrick, 1974; Rankin, Buck, and 

 Foerster, 1974). The absence of any similar trend toward an increase 

 in bio volume at the discharge station (1904) as compared with the 

 intake station (3) further supports this interpretation. The propor- 

 tion of samples exhibiting this trend was small, however, compared 

 with the approximately two-thirds of the samples that showed no 

 significant difference. 



An increase in both spatial and seasonal homogeneity was shown 

 in the distribution of dominance by phytoplankton classes and taxa 

 for the operational years. In years 5 and 6 the Chlorophyceae, 

 primarily Nannochloris and Chlorella, were dominant by density in 

 all samples. By biovolume, the Dinophyceae, represented by 

 Peridinium, became the major dominant of all seasons except winter 

 in year 5 and of all seasons in year 6. Almost all samplings showing 

 localized distribution of class or taxon dominance were limited to 

 the base-line years. Because of power-plant operation, the mixing of 

 surface waters in the different regions of the lake created more 

 uniform conditions and a more even distribution of populations. 

 Thus high frequencies of dominance would be expected from the 

 best-adapted taxon. This occurred dramatically during the first 

 month of pump operation when a one-time bloom occurrence of 

 Fragilaria was spread to every station in the lake. Thereafter there 

 was a trend toward fewer taxa attaining the status of a principal 

 dominant. 



The frequency-of-dominance differences occurring between 

 base-line and postoperational years were not similarly reflected in 

 any notable density or biovolume changes. Although autumn-to- 

 spring dominance by Bacillariophyceae, represented primarily by 

 Melosira, decreased markedly after year 4, there was no corre- 

 spondingly great change iii diatom abundance. The warmer winter 

 water temperatures, caused by both the warming climatic trend and 

 the added thermal discharges, may have favored algae more adapted 

 to warmer temperatures than the diatom taxa. (In fact, preliminary 

 data from year 7 indicate that the cold winter of 1977 was followed 

 by a return of Melosira as an important dominant.) 



The shift in increasing dominance to the principal genera 

 Nannochloris by density and Peridinium by biovolume began during 



