594 LENAT 



sity, and evenness. The total number of organisms collected at M 

 stations was maximal during year 4, but numbers at D stations 

 continued to increase through year 6. The high numbers of benthic 

 organisms collected at this time were due largely to very high 

 densities of Naididae collected at D stations in the summer and fall 

 of year 6. 



Average taxa richness varied little during the course of the study. 

 Average values for both D and M stations were in the range of 27 to 

 30 taxa. Discharge stations were slightly lower than M stations during 

 all years. Maximum numbers of taxa (up to 55) occurred at 

 headwater areas where both pond and stream organisms were 

 occasionally collected. One particular value appeared to be unusually 

 low, that at station 1 in January of year 5. Only nine taxa were 

 collected, but at nearby stations 25 to 35 taxa were collected. Only 

 one chironomid species (Psectrocladiiis) was present at this time. The 

 type of substrate present (the macrophyte Potamogeton) was not 

 sufficient to explain this anomalous value. The low number of taxa 

 suggested some toxic effect of ash basin discharge, possibly as a 

 "slug," which occurred as a one-time event. 



Diversity and evenness are always greater at M stations. This 

 difference increased in years 5 and 6 but was not great enough to 

 suggest a highly significant difference. 



Approximately 40% of the dominant littoral taxa showed 

 changes in density that were associated with increases in tempera- 

 ture. Many of these changes could be detected only because of the 

 large number of stations sampled, however. The small changes in 

 diversity and between-group similarity suggest that the changes had a 

 relatively minor impact on the total community. 



ACKNOWLEDGMEIMTS 



The research reported here was supported by a grant from Duke 

 Power Company, Charlotte, N. C. Charles M. Weiss and Terry P. 

 Anderson contributed to the development of the ideas presented in 

 this manuscript. 



REFEREIMCES 



Aston, R. J., 1973, Field and Experimental Studies on the Effects of a Power 

 Station Effluent on Tubificidae (Oligochaeta: Annelida), //ydrob/o/o^/a, 42: 

 225-242. 



Berg, C. I., 1950, Biology of Certain Chironomidae Reared from Potamogeton, 

 Ecol. Monogr., 20: 84-101. 



