810 BUIKEMA et al. 



Heinle (1976) and Lanza et al. (1975) noted that copepod mortality 

 was caused primarily by chlorine and that elevated temperature and 

 mechanical stress had little effect on mortality. Davies, Hanson, and 

 Jensen (1976) also found that elevated temperature and entrainment 

 had little effect on copepod mortality. However, Carpenter, Peck, 

 and Anderson (1974) noted that hydrauhc stress was the major 

 factor contributing to copepod mortality, accounting for 70% 

 mortality. 



Astrauskas and Rachyunas (1975) noted ~50% reduction in a 

 zooplankton population after entrainment, but Buss (1975) and 

 Benda and Gulvas (1976) stated that entrainment accounted for less 

 than 15% mortality in Lake Michigan zooplankton. Most of this 

 mortality was attributed to mechanical stress. Storr (1974) also 

 noted that mechanical stress accounted for a major part of total 

 mortality. Davies £uid Jensen (1975), summarizing a study of three 

 mid-Atlantic power plants, concluded that zooplankton mortality 

 was not affected by mechanical stress per se but by a combination of 

 ambient temperature, temperature rise, mechanical stress, and 

 chlorination levels. 



Freshwater cladocera appear to be the most sensitive of the 

 zooplankters to entrainment (Bunting, 1974); marine cladocera 

 seemingly are not affected (Markowski, 1960; 1962). Mechanical 

 stress appears to be the most important factor for large cladoceran 

 mortality (Harmsworth, 1974) although temperature may be a 

 limiting factor for some species (Whitehouse, 1971). The effect of 

 chlorination is well known (see, e.g., Brungs, 1973), and its principal 

 effects were on mortality and reproduction of Daphnia magna. 



The primary purpose of our research was to look at the 

 interactions of thermal, chlorine, and mechanical stresses in the 

 biology of a representative cladoceran, Daphnia pulex. The results 

 may be of value in identifying research tools to quantify the impact 

 of entrainment on zooplankton. 



MATERIALS AIMD METHODS 



Daphnia pulex cultures were maintained at 20°C and 150 lux 

 intensity under cool-white fluorescent bulbs. Culture and test water 

 was soft (40 ppm hardness), carbon-dechlorinated, Blacksburg, Vir- 

 ginia, tap water. Daphnia cultures were fed ad hbitum with log phase 

 Chlamydomonas reinhardii (wild, —strain), and the algae were 

 cultured in Bold's basal medium. 



Simulated entrainment exposure was modeled after that of 

 Coutant (1970; 1974) (Fig. 1). In a 12-min period, the animals were 



