CRADDOCK: EFFECTS OF WATER TEMPERATURE ON DAPHNIA PULEX 



Table 5. — Mortality and reproduction of Daphnia pulex 

 (Seattle race) exposed as maturing females for 15 min to vari- 

 ous temperatures and returned to acclimation temperature 

 of 15°C. 



'Days in parentfieses. 

 ^Total reproduction for 90 days of test. 

 ^Average daily reproduction per surviving adult. 



■•Produced by Ifie one survivor of tfie 15-min exposure during the succeed- 

 ing 90 days. 



min. One hour after exposure, one animal had 

 died at 33°C, but TD50 took 792 h (33 days) at 

 33°C and 1,008-1,536 h (42-64 days) after expo- 

 sure to temperatures below 33°C. Time in days to 

 reach TD50 was not statistically significant (x^ = 

 6.89, 5 df ) for temperature treatments of 15° to 

 30°C. A temperature in excess of 30°C for the 

 15-min exposure was necessary to significantly 

 increase mortality. 



The rate of reproduction was not significantly 

 changed by an exposure of 15 min to increased 

 temperatures through 30°C (x^ = 0.79, 5 df ). The 

 greatest total reproduction was by those D. pulex 

 tested at 24°C (Table 5) where survival was also 

 good. Reproduction at 27° and 30°C exceeded the 

 reproduction at 19°C, so it appears that reproduc- 

 tion is not materially affected by a short exposure 

 to temperatures through 30°C that do not seri- 

 ously affect survival. Reproduction by animals 

 tested at 33° and 36°C was drastically reduced 

 because most of the test animals died. 



DISCUSSION 



In zooplankton sampling of the Prescott- 

 Kalama section of the Columbia River in 1968-69, 

 D. pulex was more abundant during periods of 

 higher water temperature (Craddock et al. see 

 footnote 2). Numbers of D. pulex were low during 

 the portion of the year when the temperature re- 

 mained below 15°C (late fall, winter, and spring), 

 but as water reached and exceeded this tempera- 

 ture the population increased rapidly until the 

 peak abundance was reached at the maximum 

 water temperature (approximately 21°C). The 

 mean daily water temperature in August (the 



month of highest temperature) ranged from 19.3° 

 to 22.8°C in 1968 and from 19.7° to 21.1°C in 1969 

 (Snyder and McConnell 1971). Tauson (1931) 

 found temperatures of 16°-22°C favorable for 

 parthenogenetic reproduction by D. pulex, but 

 above or below this range production was reduced 

 considerably. The upper limit was 30°C. Ivleva 

 (1969) reviewed literature on the thermal range 

 of Daphnia and noted that several researchers 

 reported the optimum temperature range for de- 

 velopment of D. pulex as 18°-20°C. Ivleva made 

 the general observation that the optimal range 

 varies with age and the young are more resistant 

 to high temperature than the old, as was indi- 

 cated by my experiments. Other researchers re- 

 viewed by Ivleva found that mass mortalities 

 could occur in the range of 28°-32°C. Some of 

 these researchers indicated that when Daphnia 

 species are acclimated to higher or lower temper- 

 atures over a long period they become more 

 resistant to further increases or reductions in 

 temperature. 



My experiments to determine the effect of in- 

 creased temperature on D. pulex that were 1 wk 

 old and 1 day old (i.e., at the start of the experi- 

 ment) indicated that the younger animals 

 adapted better to increased temperatures. Tem- 

 peratures of 21°C and above seriously reduced the 

 length of survival of the older females (21°C = 

 TD50 in 259 h), whereas temperatures of 24° to 

 27°C or more were required to have the same ef- 

 fect on the younger females (24°C = TD50 in 648 

 h; 27°C = TD50 in 504 h). Temperatures above 

 27°C caused TD50 in a short time (less than 21 h) 

 for both age-groups. 



Although the younger females survived better 

 at the control and lower test temperatures (15°, 

 18°, and 21°C), their eventual production of 

 young was considerably less than that of the ma- 

 ture animals. This difference was not due solely 

 to the 1-wk difference in age, and I do not have an 

 adequate explanation. 



My experiment comparing survival and repro- 

 duction of the Seattle and Columbia races indi- 

 cated that the Columbia Dap/?7zm may be less re- 

 sistant to increased temperatures. The results of 

 the tests of Seattle D. pulex acclimated at 15° and 

 20°C are not directly comparable and, although 

 there is some indication that the higher tempera- 

 ture acclimation increases resistance in the mid- 

 range (23°-24°C), the effect was not apparent in 

 the high range (26°-27°C) and no conclusion could 

 be made. 



407 



