EFFECTS OF POWER-PLANT OPERATION ON ZOOPLANKTON 635 



Several species of Ceriodaphnia were found in Belews Lake. The 

 most important were C. lacustris and C. quadrangula. The seasonal 

 distribution of Ceriodaphnia was much more limited than that of 

 Bosmina (Fig. 4). One peak generally was found to occur during the 

 warm period of the year. In year 4 this peak occurred in July. 

 However, in year 5 peaks were noted in July followed by another in 

 June. This resulted in two peaks in year 5 and inflated the yearly 

 average for this year. The June peak in year 5 was thought to be the 

 annual warm-season peak and accounts for the significant decline in 

 year 6. Smyly (1974) has presented 12 years of data on the seasonal 

 distribution of Ceriodaphnia quadrangula in two English lakes. The 

 maximum density encountered in any one year appeared to be 

 similar, although the timing of peaks varied considerably between 

 years. 



Ceriodaphnia was relatively absent from Belews Lake during 

 year 6. This decline does not appear to be due to temperature. 

 Carlson (1974) showed that Ceriodaphnia was resistant to tempera- 

 tures up to 42°C. In addition, it was the second most abundant 

 cladoceran found in Lake Hyco, North Carolina, a cooling reservoir 

 for a coal-fired power plant which had surface temperatures of 40° C 

 (Weiss et al., 1975a). The Ceriodaphnia peak reappeared in the spring 

 of year 7 in Belews Lake. We will analyze its distribution more 

 critically in a future report. This reappearance shows that it was not 

 permanently affected by whatever factors caused its decline. 



Increasing Taxa 



Four taxa showed a significant increase from year 4 to year 6 

 (5% level): Hexarthra spp., Ptygura spp., Ploesoma truncatum, and 

 Synchaeta spp. It is interesting to note that two of the taxa 

 (Hexarthra and Ptygura) are warm stenotherms. 



Hexarthra spp. 



Figure 7 had a temperature optimum in Belews Lake of 24 to 

 34°C. Maximum densities were recorded in August of year 6 at 

 station 7 (32.3°C). Similar summer peaks have been recorded at Lake 

 Hyco and the lower Catawba lakes, North Carolina (Weiss etal., 

 1975a; 1975b), Lake Anna, Virginia (Saunders, 1975), and Smith 

 Mountain Lake, Virginia (Loeffelman, 1976). Edmondson (1960) 

 found that Hexarthra fennica population densities were highly 

 temperature dependent, and it was favored by high temperatures. 

 Hutchinson (1967) reviewed this information and stated that "it is 

 not unlikely that opportunistic species appearing at the height of the 

 summer would exhibit marked temperature dependence." 



