512 



LOZANO AND KITCHELL 



ture dependent. Strong (1972; 1973) showed that there are 

 inheritable differences among populations of H. azteca from locali- 

 ties with different thermal and predatory characteristics. In our 

 study, we detected changes in distribution, growth, and abundance 

 of H. azteca which were attributable to the thermal characteristics of 

 a heterothermal system. This system has a temporal and spatial 

 thermal gradient different from that of nearby natural waters. 



STUDY SITE AND TEMPERATURE CHARACTERISTICS OF 

 LAKE COLUMBIA 



Lake Columbia, Columbia County, Wisconsin, is a 190-ha 

 closed-cycle cooling lake owned by the Wisconsin Power and Light 

 Company. The lake is contained by dikes, which were erected on a 

 sedge meadow. A central dike separates the east and west arms of the 

 lake (Fig. 1). Water depth is kept between 1.9 and 2.4 m, at an 

 average of 2.1 m. Water losses from seepage and evaporation are 

 made up from the nearby Wisconsin River; makeup water is pumped 

 into a settling basin before it enters the main body of the lake. 



OUTFAL 



INTAK 



100 



1 I 



500 

 1 



1000 

 I 



Scale, m 



SETTLING 

 BASIN 



Fig. 1 Map of invertebrate sampling stations (1 to 10) on Lake 

 Columbia, Wisconsin. 



Cooling water is removed from the lake at the intake channel and 

 pumped through the power-plant condensers, where the temperature 

 is raised 15°C. Heated effluent flows from the outfall channel and 

 circulates around the center dike back to the intake channel in 

 approximately 5 days. Heat is dissipated to the atmosphere at an 

 exponential rate from outfall to intake channel. Because of the lake's 



