612 CAMPBELL 



trophic lake, according to the trophic levels set forth by Weiss et al. 

 (1978b). 



Mean yearly temperatures increased in the lake over the 4 years. 

 Although there was a greater increase in mean water temperature 

 during the two years of power-plant operation, at no sampling in the 

 4 years did the mean temperature exceed 32° C, and no measurement 

 of the maximum water temperature at the discharge station ever 

 exceeded 35° C. The optimum temperature for most green algae 

 seems to be between 30 and 35°C, and that for many blue-greens is 

 >35°C, whereas most diatom species prefer lower temperatures 

 (Cairns, 1956; Patrick, 1969; Canale and Vogel, 1974). Thus, since 

 the temperatures in Belews Lake, even during power-plant operation, 

 were not great enough to actively select for blue-green algae, green 

 algae would be expected to be the dominant class in the warmer 

 seasons. In fact, the Chlorophyceae was the most frequent class by 

 density during all seasons in all 4 years. Abundant blue-green algal 

 densities were limited to July of the two base-line years. Blue-greens 

 are favored by increases in phosphorus (Welch, Hendrey, and StoU, 

 1975); thus the absence of any blue-green dominance in summer 

 during power-plant operation was presumably caused by the very low 

 nutrient supply. 



Lowest seasonal water temperatures occurred in winter. These 

 values increased with succeeding years, because of both a general 

 climatic warming trend and the thermal input during the postopera- 

 tional years. Temperature is considered to control phytoplankton 

 populations in winter and early spring (Yentsch et al., 1974), and 

 increases of ~8°C in temperatiire were reported to stimulate 

 phytoplankton production when natural water temperatures were 

 16° C or cooler (Morgan and Stross, 1969). In Belews Lake, however, 

 the discharge-receiving stations exhibited no apparent total phyto- 

 plankton density or biovolume increases over the intake stations in 

 the cooler seasons. 



Comparing the circulation patterns at the four stations revealed 

 an increase in spatial homogeneity of the phytoplankton after 

 power-plant operation commenced. In the postoperational period 

 there was a 9% increase in the number of adjacent-station pairs with 

 no significant difference in density over the number in the base-line 

 period and a 30% increase in pairs with a difference of less than 300 

 mm^ /m'^ in biovolume. With the circulation aiid mixing effects by 

 the currents generated, phytoplankton patchiness decreased, and 

 abundances became more uniformly distributed throughout the lake. 



Where there were significant density differences in the adjacent- 

 station comparisons, there was a tendency toward greater density at 



