602 CAMPBELL 



periods of thermal discharge. The major influence of the surface 

 thermal plume was at stations 1904 and 5 in the discharge arm. The 

 influence extended, to a lesser degree, to both stations 7, farther up 

 the discharge arm, and 1907, on the main lake adjacent to the 

 discharge canal connecting the two arms. From station 1907 

 influence of the surface plume was sometimes found to reach 

 upstream as far as station 6 and downstream in the circulation 

 current to 1906. Calculations based on the average monthly pumping 

 rates indicated that in the first year of operation the pumps had the 

 capacity to circulate a volimie of water equivalent to the epilimnion 

 (7.6 m depth) in 5.5 weeks. This circulation time was reduced to 

 3 weeks after addition of the second generating unit in 

 December 1975. 



Total Phytoplankton Standing Crop 



The means for five parameters used to assess various aspects of 

 total phytoplankton standing crop are presented in Table 1. Mean 

 total cell densities and enrichment indexes each decreased through 

 the 4 years, and numbers of taxa per sample and Shannon — Weaver 

 diversity indexes steadily increased. 



The seasonal distributions of mean total density and biovolume 

 values for Belews Lake are presented in Fig. 2. A biovolume peak in 

 June 1974 stands out sharply here because of exceptional abun- 

 dances of the filamentous pennate diatom Fragilaria crotonensis. If 

 this month's values are not included in the averaging, mean total 

 biovolume values show a slight but steady increase over the 4 years. 

 The maximum mean density value occurred in July 1972, as a result 

 of a great abundance of the minute colonial blue-green alga 

 Agmenellum quadhduplicatum. Abundances of this blue-green alga 

 alone were responsible for the very large enrichment indexes of 

 year 3. 



Minimum temporal mean values for total density, total bio- 

 volume, and total number of taxa per sample all occurred in the 

 coldest period of each year, between December and March. Even 

 during power-plant operation, there was no significant winter 

 increase in total phytoplankton density or biovolume at stations 

 influenced by the heated water. Spatially the maximum mean values 

 for these three parameters occurred predominantly at the upstream 

 stations nearest the nutrient supply provided by inflowing streams. 

 The largest range in values for total density, biovolume, enrichment 

 indexes, and Shannon— Weaver diversity indexes all occurred in 

 year 3, when the lake was still filling. 



