624 ANDERSON AND LENAT 



ture vs. depth profiles under stratified conditions are characterized 

 by a warm layer (near surface temperatures) 4 to 6 m deep 

 overlaying progressively cooler water which may include a portion of 

 the metalimnion. From November to March the average main lake 

 euphotic-zone temperatures (except for station 1904) vary less than 

 1 to 2°C from the surface temperatures owing to destratifying forces. 

 The surface temperatures discussed in the following paragraphs are 

 meant to give an idea of the general impact of power-plant operation 

 on the temperature regime of the lake. 



Average surface temperatures for 11 stations are given in Tables 1 

 and 2. Average surface temperatures increased from year 4 to year 6 

 (from 18.9 to 23.6°C), but this increase was not uniformly 

 distributed over all months. Power-plant operation affected the 

 temperature regime in two ways. First, winter temperatures in- 

 creased. Winter temperatures in year 6 were 5 to 12°C greater than in 

 year 4. There was a steady increase in the minimum temperature 

 recorded from 7.5°C in year 4 to 13.0°C in year 6. A further effect 

 of temperature increases has been a decrease in the annual 

 teraperature fluctuation, as reflected in the decreasing standard 

 deviation of yearly average temperatures. Second, the period of 

 maximum temperature increased, especially in year 6, although the 

 maximum temperature remained approximately the same. Average 

 surface temperatures greater than 30°C were recorded in only 

 1 month in year 4 but were observed in 3 months in year 6. The 

 variation in mean air temperatures recorded at the Greensboro, N.C., 

 airport (National Oceanic and Atmospheric Administration records) 

 over the natural warming period (January through August) between 

 years was less than 1°C. The contribution of natural heating to 

 surface temperature was therefore thought negligible compared to 

 that of the power plant. 



It is important to note periods when average surface tempera- 

 tures did not change appreciably. In early fall and late spring of 

 year 5, the power plant was shut down. Comparisons between 

 years 4 and 5 during these periods indicate that there was little 

 systematic change in temperature; changes in the seasonal pattern of 

 fall— spring species therefore should be observed mainly in year 6, 

 although some shifts might be seen in the early spring of year 5. 



Phytoplankton 



Changes in food concentrations and food quality in Belews 

 Lake were assessed by examining algal standing crop. Data on algal 

 densities are given in Fig. 3. The line for total density primarily 

 reflects the distribution of nannoplankton (Nannochloris, Chlorella, 



