EFFECTS OF POWER-PLANT OPERATION 601 



total for the sample was then divided by the total number of taxa 

 found (Weiss et al., 1974). Diversity was measured by the Shannon- 

 Weaver index (Shannon and Weaver, 1949; Lloyd and Ghelardi, 

 1964). 



RESULTS 



Physical and Chemical Parameters 



Mean Secchi disk values exhibited a general trend toward 

 increasing depth over the 4 years, from 1.6 m in year 3 to 2.6 m in 

 year 6. Nutrient levels in the lake were low. Yearly mean inorganic 

 nitrogen levels decreased steadily from 127 jug/liter in year 3 to 

 67 /Jg/liter in year 6. Soluble reactive phosphorus levels were very 

 low, with maximum values never exceeding 10 /ig/liter, and mean 

 values exhibited a steady decrease from 8.5 yug/liter to 2.3 //g/liter 

 through the 4 years of study. This trend was at least partially due to 

 refinements made in the analytical technique during this period. 

 Nutrient inputs to the lake were primarily associated with periods of 

 high runoff. 



The mean yearly surface-water temperatures measured during 

 sampling increased for each of the 4 years of study, from 17.5 and 

 18.9° C in the base-line period to 21.0 and 23. 6° C in the postopera- 

 tional years. Although this increase paralleled a general warming 

 trend in the weather over this period, greater increases in mean water 

 temperature occurred during the 2 years of power-plant operation. 

 The mean water temperature of the warmest month was between 

 30.0 and 31. 5° C in all years. During the 2 years of power-plant 

 operation, the highest water temperatures encountered occurred in 

 summer at the discharge station (1904). Temperatures at the time of 

 sampling approached but never exceeded 35.0° C. Temperature 

 differences between the intake station (3) and the discharge station 

 (1904) during the 19 samplings when the power plant was in 

 operation averaged 6.7° C, with no difference as great as 10.0°C. 



The pumps that circulate lake water through the power-plant 

 condensers began operation in June 1974, and in July the currents 

 began carrying the thermal discharge from the plant. The main 

 circulation pattern generated by the power plant ran from the intake 

 near station 3 through the condensers to station 1904, then through 

 the connecting discharge canal to 1907, down this main arm of the 

 lake to 1906, and up the intake arm back to station 3 (Fig. 1). 

 Except for September 1974 and the period from April to June 1975, 

 all samplings for the two postoperational years occurred during 



