822 BUIKEMA et al. 



nominal impact on egg production because the number of eggs 

 produced is size dependent. Because there were no significant effects 

 on reproduction, the effect of chlorine on body size was considered 

 insignificant. 



Even though we did not study the interactions of two specific 

 stresses, we felt that the effects of combined stress on cladoceran 

 functions probably reflected only the action of specific stress. For 

 example, the significant decrease in ATP levels may be caused by 

 chlorine, and the poor survival of first instars may be caused by 

 mechanical stress. That there was no significant effect of combined 

 stress on oxygen consumption suggests that the stimulatory effect of 

 mechanical stress was negated by the inhibitory effect of thermal 

 stress. 



Generally, entrainment caused a decrease in field populations of 

 zooplankton (see, e.g., Astrauskas and Rachyunas, 1975), probably 

 because of the "predatory" impact of mechanical stress (Bunting, 

 1974). On the basis of our data, adult Daphnia pulex have a greater 

 chance of surviving entrainment stress than do young Z). pulex unless 

 the thermal shock approaches 35°C, the incipient lethal for Z). pulex 

 (Goss and Bunting, 1976), or unless exposure to chlorine exceeds 

 0.5 ppm for more than 30 sec without subsequent dilution. 



Evaluating the response of surviving individual organisms to 

 entrainment, we find no measurable effect on their biology if food is 

 healthy and in sufficient quantities. If food was limiting in any way, 

 latent effects of thermal stress to the Daphnia became apparent, and 

 these effects contribute to the depressed population numbers 

 observed below the discharge areas. The effects are further increased 

 because damaged and scenescent algae inhibit feeding responses of 

 cladocera (Ryther, 1954). 



The significant increase in zooplankton abundance in heated 

 discharges observed by Goryajnova (1975) and others may be caused 

 not by the thermal effects on entrained animals but by a combina- 

 tion of recruitment of organisms not previously entrained and the 

 effects of variable temperatures on reproductive potential. Bunting 

 (1974) and Swain, Wilson, and Neri (1975) showed that higher 

 temperatures decreased development time and longevity while 

 increasing reproduction rates. Feeding rates also increased with 

 increasing temperature (Duval and Geen, 1976; Hay ward and Gallup, 

 1976), and this would affect reproduction. Buikema and Loeffelman 

 (1977), Halbach (1973), and Ruttner-Kolisko (1964) demonstrated 

 that fluctuating temperatures increased the reproductive potential of 

 rotifers, and similar responses would be expected for other zoo- 

 plankton. 



