LITTORAL BENTHOS OF BELEWS LAKE 591 



of maximum temperature in year 7 might well cause a drop in the 

 standing crop of Hexagenia in the discharge basin. Additional 

 samples were taken at stations 7, 5, and 1904 on Sept. 22, 1976, to 

 test this hypothesis (part of year 7 work is in progress). Data from 

 years 5 and 6 indicated that maximum densities should occur during 

 this period, but no specimens of Hexagenia could be found at any of 

 these stations. Samples taken from stations 1907 and 8, however, 

 indicated that Hexagenia was still present in the main lake basin in 

 considerable numbers (-^lOO/m" ). 



Limnodrilus spp., principally L. hoffmeisteri (OHgochaeta), 

 showed a distinct maximum at discharge stations in year 5. This was 

 the only period in which a significant difference could be observed 

 between M and D stations. These data suggest that the year-5 

 temperature regime at D stations was optimal. 



Tubificidae, especially Limnodrilus, have frequently been shown 

 to have a positive response to increases in temperature (Boltovskoi, 

 1970; Weiderholm, 1972; Aston, 1973; Gallup, Hickman, and 

 Rasmussen, 1975). This was the pattern observed at Belews Lake 

 between years 4 and 5. Aston (1973) demonstrated a more complex 

 pattern in laboratory cultures. He found that egg production 

 increased up to 25°C but declined at 30°C. Egg production at 30°C 

 was approximately equal to that measured at 15 to 20° C. These 

 temperature data are not directly comparable to Belews Lake since 

 constant temperatures were used, but the general pattern is analo- 

 gous to the overall distribution seen for years 4 to 6. 



Several taxa appear to have been indirectly affected by increases 

 in temperature. Increases in the densities of Dero/Nais sp. (Oligo- 

 chaeta) and Palpomyia (complex) (Diptera) were associated wdth 

 growths of filamentous algae (probably Anabaena) at D stations in 

 the end of year 5 and the beginning of year 6. Thermal effluents 

 have previously been shown to stimulate periphyton growth, 

 especially for blue-green algae (Foerster, Trainor, and Buck, 1974; 

 Hickman, 1974; Bush, Welch, and Mar, 1974). 



Three organisms developed maximum densities at M stations 

 during the postoperational periods, Cyrnellus marginalia (Trichop- 

 tera), Endochironomus nigricans (Chironomidae), and Eukiefferiella 

 sp. (Chironomidae). These organisms had low densities at all stations 

 during year 4 and remained at low densities at D stations through 

 year 6. This distribution parallels the development of macrophytes in 

 the lake. 



Endochironomus nigricans has frequently been associated with 

 macrophyte growths (Berg, 1950; Buckley and Sublette, 1964; 

 Lindeman, 1942). Macan (1975) indicated that the optimal habitat 



