EFFECTS OF POWER-PLANT OPERATION ON ZOOPLANKTON 633 



ConocJiilus unicornis showed a significant increase from year 4 to 

 year 6. However, missing data in year 6 invalidates this comparison, 

 and this species is considered stable. We observed, however, that the 

 period of minimum densities decreased from 5 to 4 to 2 months as 

 winter sm'face temperatures increased (Fig. 4). Conochiloides spp. is 

 another taxon considered to be stable in spite of a statistically 

 significant decline between years 4 and 6. Two species of Cono- 

 chiloides were recorded from Belews Lake, C. dossuarius and C. 

 coenobasis. The seasonal distribution of Conochiloides (Fig. 4) was 

 appai'ently that of a warm stenotherm, but we think that this 

 organism was present in the metalimnion at much lower tempera- 

 tures. Makarewicz and Likens (1975) found that Conochiloides was 

 present only in the hypolimnion of Mirror Lake, New York, and 

 Makarewicz (1974) implied that it feeds mainly on bacteria and 

 detritus, as it occurred below the zone of phytoplankton production. 

 Bottom water of Mirror Lake was not anaerobic, and Conochiloides 

 was present throughout the hypolimnion with a maximum near the 

 bottom. In situations where the hypolimnion becomes anaerobic, 

 however, rotifers tend to concentrate at the area of oxygen 

 depletion, i.e., the metalimnion (Campbell, 1941). Conochiloides, 

 therefore, was probably found in or near the metalimnion in Belews 

 Lake because its hypolimnion is anaerobic during stratification. The 

 apparent decline in Conochiloides from year 4 to year 6 was 

 probably due to a deepening of the epilimnion, which caused 

 Conochiloides to be missed in euphotic-zone collections, rather than 

 to an actual drop in the density of this organism. This supposition is 

 supported by data from deep samples (bottom-to-surface vertical 

 hauls) in which the density of Conochiloides did not decline from 

 year 4 to year 6. Moreover, the peak observed in euphotic-zone 

 samples (August) was extended into the fall in deep samples for all 

 3 years. 



Declining Tax a 



Six taxa showed statistically significant declines in year 6 

 compared to year 4 and/or year 5: Keratella crassa, Collotheca spp., 

 Asplanchna spp., Calanoid copepodids (=Diaptomus spp.), Meso- 

 cyclops edax, and Ceriodaphnia spp. Keratella crassa was the most 

 abundant rotifer in Belews Lake during all years of this study. Little 

 is known about its ecology. Keratella crassa is usually found to 

 coexist with K. cochlearis. Makarewicz and Likens (1975) found that 

 the. maximum density for K. crassa in Mirror Lake, New York, was 

 near the surface. Keratella cochlearis in this lake had a more uniform 

 vertical distribution, with a maximum density at 3 m. A study by 



