814 BUIKEMA et al. 



using Duncan's new multiple range test. In all cases, the level of 

 significance was P < 0.05. 



RESULTS 



After 24 hr, survival of first instar and adult organisms was not 

 affected by a 10°C stress. When adults were exposed to 34°C, they 

 also survived, but 35°C (a 15°C stress) was lethal. The effect of 

 chlorine was dependent on concentration and length of exposure 

 (Table 1). A 1-min exposure to 0.1 ppm chlorine killed 50% of the 

 population within 48 hr. If food was provided immediately after a 

 22-sec chlorine exposure, the tolerance level increased to 0.5 ppm. 

 Mechanical stress accounted for 30 to 60% mortality in adults and 60 

 to 80% in first instars within 1 hr after exposure. When all three 

 stresses were combined, the percent mortality was similar to that 

 obtained for mechanical stress. 



The daphnids that survived the mechanical and combined stress 

 were held for the duration of their lives, and adult survival was not 

 significantly different from the controls (Table 2). First instar 

 organisms, however, exhibited 50% mortality within 24 hr and rarely 

 survived for more than 3 days. Chlorine and 10°C temperature stress 

 did not affect long-term survival. 



Surviving first instar and adult organisms were monitored after 

 exposure for the duration of their lives. As long as healthy food was 

 provided, there was no significant effect of individual or combined 

 stresses on survival, molting rate, number of live young per brood 

 (Table 2), or number of deteriorated eggs. Growth was not affected 

 by stress in adult animals but was significantly reduced in chlorine- 

 stressed first instar organisms. 



If food was limiting in any way, however, latent effects of stress 

 were observed. Significant depressions in reproduction occurred if 

 the daphnids were fed trout chow or scenescent algae. Figure 2 

 shows the effect of scenescent algae fed between the seventh and 

 eleventh instars. The thermally shocked daphnids had significantly 

 lower numbers of live young immediately after being fed scenescent 

 algae and recovered more slowly after being fed log-phase algae. 

 During this period both groups of animals had increased numbers of 

 deteriorated eggs per brood, and in instar eight the shocked animals 

 had significantly higher numbers of deteriorated eggs (18 vs. 4 for 

 the controls). In a subsequent study, Daphnia exposed to the 

 combined stresses were fed unstressed log-phase algae, algae stressed 

 by temperature and chlorine, and a 50 : 50 mixtiu"e of stressed and 

 unstressed algae (Table 3). In most instances stressed food signifi- 



