EFFECTS OF POWER-PLANT OPERATION ON ZOOPLANKTON 619 



important factors controlling zooplankton abundance (Edmondson, 

 1965; Hayward and Gallup, 1976). 



Temperature has been shown to affect the reproductive rates and 

 seasonal cycles of both Crustacea and Rotifera. Selgeby (1975) 

 compared the seasonal cycles of Crustacea in cold Lake Superior 

 with those in warmer Lake Michigan and Lake Erie. He found that 

 differences in temperature can advance breeding 1 to 3 months. 

 Williams (1964) also has found differences in the seasonal patterns of 

 rotifers when comparing areas with different temperature regimes. 

 Another important variable affecting zooplankton densities is the 

 presence of planktivorous fish (Brooks and Dodson, 1965; Stenson, 

 1973). 



Between-year changes in the zooplankton community are most 

 probably due to changes in these three factors. We, therefore, 

 assessed zooplankton dynamics in Belews Lake in light of tempera- 

 ture, food, and planktivorous fish populations. Power-plant opera- 

 tion may have superimposed a fourth factor. Our main objective has 

 been to distinguish power-plant effects from those due to natural 

 causes. We have placed our emphasis on lake-wide effects instead of 

 concentrating on local areas of impact, such as the discharge cove 

 area. We believe that this is the proper focus of concern since 

 localized studies inevitably lead to questions about what the results 

 mean in terms of the functioning of the system as a whole. 



STUDY AREA 



Belews Lake (Fig. 1) was constructed by Duke Power Company 

 as a cooling reservoir for the coal-fired Belews Creek Steam Station. 

 The lake lies in the Piedmont section of North Carolina. It has a 

 surface area of 1500 ha, a retention time of approximately 

 1000 days, and a mean depth of 15 m (maximum depth —40 m). 

 Belews Lake has been classified as oligomesotrophic (Weiss and 

 Kuenzler, 1976). 



The Belews Creek Steam Station has two operating units, each 

 with a capacity of 1145 MW. Operational data for years 5 and 6 are 

 given in Fig. 2. The mean pumping rate of 52 mVsec in year 6 is 

 theoretically capable of circulating the entire epilimnion of the lake 

 (average depth of 7.5 m) in 3 weeks. Note that several periods of 

 shutdown occurred, especially in year 5. 



Belews Lake has been the subject of an environmental study for 

 7 years, designated year 1 to year 7. Year 1 (August 1970 to April 

 1971) was a pre impoundment study with dam closure at the 

 beginning of year 2 (May 1971 to June 1972). Lake filling continued 

 through the end of year 3 (July 1972 to June 1973). Year 4 (July 



