382 WELCH AND WARD 



additions on species or on populations, but not a great deal is known 

 about the effects at the community, level. When the thermal 

 optimum for existing species is exceeded in natural waters, the 

 species are unable to compete for the available resources, and the 

 community structure of the system changes. The seasonal variation 

 of resources superimposes more complicating factors. The question 

 is, Will system function, energy flow, and productivity also be 

 altered? 



The objectives of this study were to measure and evaluate the 

 effect of increased temperature on a phytoplankton community as 

 indicated by primary productivity. Annual primary productivity of 

 the heated and unheated surface waters of a cooling reservoir were 

 investigated in situ by ' '^C methods. 



LITERATURE 



Warinner and Brehmer (1966), studying the effects of thermal 

 effluents on a community of marine organisms in a riverine estuary, 

 found an increase in primary production during the winter months 

 and a decrease during the summer months. Patrick (1969) noted 

 that, as long as nutrients and light are sufficient, productivity may 

 increase with increases of temperature within the thermal tolerance 

 range of the existing algae. Simmons and Armitage (1974) deter- 

 mined that heated power-plant effluent had no effect on algal 

 blooms in the Potomac River. Foerster, Trainor, and Buck (1974), 

 on finding that blooms correlated well with increased temperature, 

 thought that the mechanism might be an increase in the rate of 

 diffusion across the depletion zone surrounding the algal cell. Tilly 

 (1973; 1974; 1975) and Marshall and Tilly (1971) reported on 

 investigations of phytoplankton and periphyton in Par Pond in South 

 Carolina. Maximum productivity and integral productivity were 

 strongly correlated with temperature increases, but primary produc- 

 tivity per unit of chlorophyll was not. Tilly theorized that elevated 

 temperatures enabled phytoplankton to use higher light intensities 

 without photoinhibition, a phenomenon well documented in labora- 

 tory studies (Sorokin and Myers, 1953). 



DESCRIPTIOIM OF THE STUDY AREA 



Lewis Creek Reservoir (Fig. 1) is a 404-ha reservoir constructed 

 in 1970 by Gulf States Utilities as a semiclosed system to cool the 

 condensers of a 530-MW electric generating plant. The reservoir, 



