586 LENAT 



according to the criteria established. For ten of these organisms, 

 changes in density were found to be associated with increases in 

 temperature (i.e., criterion 4). Data for these organisms are presented 

 in Tables 2 and 3. 



Five organisms appeared to be directly affected by increases in 

 temperature. Decreases in densities of three of these organisms 

 (Polypedilum digitifer, Nais communis, and Hyalella azteca) were 

 associated with increased temperature. Densities of the other two 

 organisms (Limnodrilus hoffmeisteri and Hexagenia munda) in- 

 creased with slight increases in temperature but declined at higher 

 temperatures. 



Polypedilum digitifer (Chironomidae) was a major component of 

 the littoral benthos over the entire course of the study. During the 

 course of this study, the density of P. digitifer remained remarkably 

 stable at M stations. At D stations, however, an order of magnitude 

 decline was noted from year 4 to year 5. The density of P. digitifer 

 was significantly greater at D stations than at M stations in year 4, 

 but this situation reversed in year 5. No significant chaiiges occurred 

 after year 5. This pattern suggests a change associated with 

 power-plant operation. 



Temperature has been shown to be one of the most important 

 factors regulating the growth rate of Chironomidae (Konstantinov, 

 1958). Temperatures in the range of 30 to 37° C have been shown to 

 be unfavorable or fatal (Curry, 1968; Nebeker, 1973; Gallup, 

 Hickman, and Rasmussen, 1975). The maximum critical temperature 

 shows considerable geographic variation, however. Temperatures 

 reported as fatal in cold temperate areas are frequently encountered 

 by organisms in subtropical regions. The only data available for 

 subtropical areas are from a study of a thermal lake in New Zealand 

 by Forsyth and McColl (1974), who found that 34°C was the critical 

 temperature for Chironomidae. 



Nais communis (Oligochaeta) was found to have maximum 

 densities in winter and spring. Between years 4 and 5, a parallel 

 increase in density was noted at both M and D stations. In year 6, 

 density continued to increase at M stations but declined at D 

 stations. Neither change was statistically significant, but a significant 

 difference between M and D stations was noted for the first time in 

 year 6. This is attributed to an increase in winter temperatures. Nais 

 communis was the only member of the family Naididae which 

 appeared to be negatively affected by high temperatures and also 

 seemed to be the only naidid occurring during cold winter months. 



The density of Hyalella azteca (Amphipoda) changed quickly 

 with the start of power-plant operation in year 5; H. azteca 



