812 BUIKEMA et al. 



Corp.). Preliminary bioassays were conducted to determine the 

 relationship between chlorine concentration and exposure time. 

 Mortality was monitored for up to 48 hr after exposure. In the final 

 studies the animals were initially exposed in a continuous-flow 

 system to 0.5 ppm total chlorine for 30 sec. Then the total chlorine 

 concentration was diluted to 0.25 ppm and maintained for 11.5 min. 

 After 12 min of exposure, the chlorine was diluted over a period of 

 ~20 min until there was no chlorine. The final chlorine concentra- 

 tion selected, 0.5 ppm, is lower than the maximum pulses of chlorine 

 reported for power plants. Chlorine levels above 0.5 ppm were lethal 

 after short exposures, however, and 0.5 ppm was the maximum 

 concentration that did not affect survival in chronic studies. 



Mechanical stress was applied by pumping the animals through a 

 submersible pump and 46 m of 12.5-mm-ID Tygon tubing (Norton 

 Company) at a rate of 4 liters/min. Total time of passage through the 

 system was approximately 1 min. Total time of mechanical stress in 

 the pump— tube system and turbulence in the receiving tank was 

 30 min. The mechanical stress emphasis of this research was on 

 passage of the organism through a 12.5-mm tube. The flow velocity, 

 46 m/min, was low for data presented for some power plants, and 

 the pressure, 4.84 psi, was within the minimum range reported for 

 the Indian Point power station (Lauer et al., 1974). The calculated 

 shear stress was 0.0485 psf . 



In the combined stress study, the Daphnia were simultaneously 

 exposed to a 10°C thermal, 0.5 ppm chlorine, and mechanical 

 stresses in a continuous-flow system. The times of exposure for each 

 component of the combined stress were as outlined and as presented 

 in Fig. 1. Mechanical stress and turbulence in the receiving tank 

 lasted for 30 min, the time required to dilute the total chlorine to 

 zero. 



In studies where the Daphnia were monitored for their entire life, 

 the animals were usually fed an ad libitum concentration of algae 

 after exposure. The effects of food quality were monitored in three 

 experiments. In two studies, thermally stressed animals were fed 

 trout chow and scenescent algae, and in the third, animals exposed to 

 the combined stress were fed algae at a rate of 20,000 cells/ml. In 

 this study, 10 animals were each fed healthy log-phase algae, stressed 

 log-phase algae, or a 50 : 50 mixture of stressed and unstressed 

 log-phase algae. The algae were preexposed to 10°C thermal and 0.5 

 ppm total chlorine stresses for the time periods discussed (see Fig. 1). 



Acutely exposed Daphnia were observed for mortality. The 

 criterion for death was no body movement after probing or swirling 

 of the test container. For organisms exposed to mechanical stress. 



