590 LENAT 



disappeared almost completely from the discharge basin. At U 

 stations, however, H. azteca showed no significant differences 

 between years 4 and 5. In year 6, H. azteca was not collected at D 

 stations, and a significant decline occurred at U stations. This 

 distribution pattern appears to be related to temperature. 



Data from studies of the effects of temperature on H. azteca 

 (Bovee, 1949; Sprague, 1963) indicate that 32°C is the critical 

 temperature for Hyalella irrespective of acclimation temperature. 

 Gallup, Hickman, and Rasmussen (1975), studying the effects of a 

 power plant operating on Lake Wabamun, Alberta, Canada, found 

 that the density of Hyalella at control stations was 19 times greater 

 than that at heated stations. 



The results of these studies are consistent with the data from 

 Belews Lake. Belews Lake data tend to confirm that 32° C is the 

 critical temperature for Hyalella. Hyalella reached high levels during 

 year 4 (preoperation) when natural lake temperatures rarely ex- 

 ceeded 32° C. In year 5, when temperatures greater than 32° C were 

 found only in the discharge basin, Hyalella promptly disappeared 

 from this area. In year 6, temperatures of 32 to 33° C were recorded 

 both in the main lake basin and in the discharge basin. This seems to 

 account for the sharp decline observed in Hyalella standing crop 

 despite the favorable habitat. The decline of Hyalella observed in 

 year 6 is probably to be expected in this geographic area. In the 

 Piedmont of North Carolina, Hyalella is usually confined to rivers 

 (Lenat, unpublished data), possibly because of the high temperatures 

 recorded in lakes of this area. 



Hexagenia munda is an important component of the littoral 

 community because of its large size. Ephemeroptera (mostly Hexa- 

 genia) make up approximately 20% of the biomass collected in years 

 5 and 6. 



Temperature appeared to exert a major influence on the 

 distribution of Hexagenia in Belews Lake. Initial inspection of 

 Hexagenia standing crop values (Table 3) suggested that the density 

 of Hexagenia was increased by heat addition up to a certain critical 

 temperature. Hexagenia increased at stations 5, 7, and 1907, where 

 temperatures averaged 1.5 to 3°C above ambient (years 5 and 6). At 

 stations with higher temperatures (1904B, 1904 A, and 1904), 

 however, Hexagenia occurred in relatively low densities. The depres- 

 sion of Hexagenia at high temperatures (>35 to 37°C) agrees with 

 data from Lake Hyco, North Carolina (Lenat, 1975). 



These data led to a hypothesis that Hexagenia standing crop 

 might be severely affected by further increases in the heat load. It 

 was thought that higher temperatures or an extension of the period 



