520 LOZANO AND KITCHELL 



summer reproductive periods. Amphipods in the cooling lake began 

 reproducing during the spring and fall, 3 months earlier, and 

 continued 2 months longer than did amphipods in the settling basin. 

 Although reproduction of H. azteca continued year-round in a geyser 

 hot spring with water temperatures between 12 and 40° C (Strong, 

 1972), reproduction in the cooling lake ceased at all stations when 

 temperatures rose above 28° C. Using Dendy samplers on a monthly 

 schedule may not be sufficient to measure low summer reproduction 

 rates. Hargrave (1970) found a high correlation between algal 

 production and density of//, azteca. Although it seems unlikely that 

 food could be limiting in eutrophic Lake Columbia, food quality 

 may be important. Benthic production measurements taken in the 

 summer of 1975 show an abundance of periphyton and other 

 invertebrates at all locations in Lake Columbia (Lozano, Rondorf, 

 and Kitchell, 1978). Even though food was abundant, the high 

 metabolic cost of maintenance during the warm summer months 

 could have inhibited reproduction. 



In conclusion, amphipods respond to the unique thermal 

 conditions in Lake Columbia by altering their distribution through 

 temperature-induced migrations. Reproduction and growth are in- 

 hibited by high temperatures in some regions of the lake and are 

 generally reduced during the warm summer period. 



ACKNOWLEDGMENTS 



The research reported here was supported in part by research 

 grants from the Office of Water Resources Technology (A-070-WIS) 

 and the Environmental Protection Agency (R803971020). 



We thank Anne Pilli and Ron Hall for advice and technical 

 assistance and Betsy Colburn for reviewing early drafts of the 

 manuscript. 



REFERENCES 



Andren, A., M. Anderson, N. Loux, and R. Talbot, 1976, Aquatic Chemistry, in 

 Documentation of Environmental Change Related to the Columbia Electrical 

 Generating Station, 9th Semi-Annual Report, lES Report 69, University of 

 Wisconsin, Madison. 



Bovee, E. C, 1949, Studies on the Thermal Death oi Hyalella azteca (Saussure), 

 Biol. Bull., 96: 123-128. 



Cooper, W. E., 1965, Dynamics and Production of a Natural Population of a 

 Freshwater Amphipod, Hyalella azteca, Ecol. Mongr., 35: 377-394. 



Gibbons, J. D., 1976, Nonparametric Methods for Quantitative Analysis, Holt, 

 Rinehart & Winston, Inc., New York. 



