CRADDOCK: EFFECTS OF WATER TEMPERATURE ON DAPHNIA PULEX 



Three experiments were conducted to simulate 

 thermal conditions that D. pulex might en- 

 counter. Two experiments studied the effect of in- 

 creased temperatures that might be encountered 

 in the area of a heated plant outfall, whereas the 

 third simulated the thermal conditions small or- 

 ganisms could encounter in the condenser cooling 

 system of a thermal power plant. 



The first experiment compared the effect of pro- 

 longed exposure (50-52 days) to constant temper- 

 atures of 15° (control), 18°, 21°, 24°, 27°, 30°, and 

 33°C. Test organisms were both mature females 

 and young females (at the start of the tests) of the 

 Seattle group acclimated at 15°C. There were 18 

 mature females per test temperature, 6 per test 

 jar, and 10 young females were tested per test 

 temperature and test jar. Ten Daphnia per 50-ml 

 jar were well below the number that would cause 

 harmful metabolic waste buildup or oxygen de- 

 pletion (Pratt 1943); 10 animals has long been 

 accepted as a standard for bioassays, Doudoroff 

 (1951), American Public Health Association 

 (1971), and Sprague (1973). 



The second experiment compared the effect of 

 prolonged exposure (34 days) to temperatures of 

 20° (control), 23°, 26°, 29°, and 32°C on mature 

 females of the two groups (Seattle and Columbia) 

 acclimated at 20°C. There were 10 animals per 

 test temperature and test jar. 



The third experiment subjected mature females 

 of the Seattle group acclimated to 15°C to a short 

 exposure (15 min) to temperatures of 15° (con- 

 trol), 19°, 21°, 24°, 27°, 30°, 33°, and 36°C. Test 

 organisms were then returned to acclimation 

 temperature where they were held and observed 

 for 90 days. Twelve animals were tested at each 

 temperature. 



Test animals were examined frequently to de- 

 termine the effect of increased temperatures, 

 usually hourly during the first 8 h of a test. The 

 next day or two, they were examined two or three 

 times a day and subsequently once each week 

 day. During each observation, the mortalities 

 were noted and removed, and newly born Daph- 

 nia were counted and removed. The animals were 

 assumed to be dead when they lay on the bottom 

 and there was no detectable movement of the an- 

 tennae, thoracic legs, or the post abdomen. 



Temperature effects were evaluated on the 

 basis of survival and reproduction by animals 

 tested at the various temperatures. In this study, 

 my evaluation criterion was the time at a particu- 

 lar temperature until 50% mortality; therefore, I 



use the term TD50 (time to death of 50% of the test 

 animals at a particular temperature). 



RESULTS 



Experiments Relating to Discharges 

 of Heated Water 



Seattle Daphnia Acclimated to Water of 15°C 



Death occurred rapidly for both mature and 

 young D. pulex at 33°C. Some animals in both 

 groups lost equilibrium within the first hour, 

 TD50 occurred before the third hour, and none 

 survived the fourth hour of exposure (Table 1). 

 Mature and young Z). pulex subjected to tempera- 

 tures above 27°C reached TD50 in less than 24 h. 

 Temperatures of 27°C and below required an ex- 

 posure of at least 192 h (8 days) to cause 50% 

 mortality. The younger females did not succumb 

 to moderately high temperatures (18°, 21°, and 

 24°C) as quickly as the older females. Tempera- 

 tures of 21°, 24°, and 27°C caused TD50 among the 

 older females after an average of 238 h, whereas 

 the younger females did not reach TD50 until an 

 average of 768 h. 



Table 1. — Mortality oi Daphnia pulex introduced as mature 

 and young females and maintained at temperatures of 15° to 

 33°C (Seattle race, acclimated at 15°C). 



All animals died before producing young at 30° 

 and 33°C; rate of reproduction was highest at 24° 

 and 27°C before all subjects died. Total offspring 

 produced and rate of reproduction varied for the 

 two age-groups of females tested at 21°C or below 

 (Table 2). 



First reproduction by the mature females oc- 

 curred 5 days earlier at test temperatures of 27°, 

 24°, and 21°C than at the control temperature 

 (15°C). Only one peak of production occurred at 

 27 °C before 100% mortality was reached. Repro- 

 duction at 15°C was stable with peaks occurring 



405 



