634 ANDERSON AND LENAT 



Loeffelman (1976) at Smith Mountain Lake, Virginia, compared the 

 densities of K. crassa and K. cochlearis in different parts of the lake. 

 He found that K. crassa was favored in areas where pump storage 

 operations increased vertical mixing. 



In Belews Lake a significant decline was noted when year 6 was 

 compared with the other years (5% level). This decline occurred 

 primarily at main lake stations and not at the stations receiving direct 

 heated discharge (stations 1904 and 1907). This indicates that 

 factors other than temperature caused its decline. 



The average yearly density of Collotheca spp. showed a steady 

 decline from year 4 to year 6 (Fig. 7). Collotheca appears to be a 

 warm stenotherm since peaks do not occur below 26° C. Pejler 

 (1957) showed that Collotheca libera was a warm stenotherm^ and 

 that C mutabilis also was restricted to high temperatures in most 

 lakes. Year 4 samples showed very high between-station variance. 

 Maximum densities of Collotheca were found only at main lake 

 stations, which implied that optimal conditions were confined to this 

 area. Power-plant circulation in years 5 and 6, however, tended to 

 reduce between-station variance (see later statistics) and exposed 

 organisms in surface waters to a common range of environmental 

 conditions. This change in environmental conditions may have been 

 unfavorable for Collotheca, although other factors, such as inter- 

 specific competition, could also account for its decline. 



Asplanchna spp. showed a significant decline (5% level) in year 6 

 when compared to year 4 and year 5. Year 4 and year 5 densities 

 were not significantly different. The distribution of Asplanchna spp. 

 was too irregular to deduce the cause of its decline. A possible 

 explanation is competition with Ploesoma truncatum, a predaceous 

 rotifer that increased in numbers at the same time. 



Calanoid copepodids {Diaptomus pallidas and D. reighardi) 

 declined sharply in the spring of year 5 and were largely absent in 

 year 6. This phenomenon was probably not related to the increase in 

 temperature. Diaptomus was found to be abundant in Lake Hyco, 

 North Carolina, where surface temperatures up to 40° C were 

 recorded (data reworked from Weiss et al., 1975a). 



Mesocyclops edax appeared to have disappeared in year 6, 

 however, it reappeared in July and August of year 7 in higher 

 densities than previously recorded (Lenat and Anderson, in prepara- 

 tion). The seasonal distribution for this species seems to be 

 somewhat irregular, and this peak shift in the warm season from 

 year 6 to year 7 accounted for the decline and indicates that it was 

 not detrimentally affected by power-plant operation. 



