278 R. G. Stress et al. 



fairyshrimps is resource-limited. Evidence for food limitation in 

 fairyshrimps was provided in measurements of fecundity. Kangas (1972) 

 showed that females of Branchinecta paludosa and Polyartemiella hazeni 

 in the laboratory released more eggs that did their counterparts in the 

 ponds near Barrow. 



An elaborate analysis of growth and reproduction was performed on 

 D. middendorffiana. Results from field observation and from experiment 

 and model simulation of the Daphnia populations in Barrow ponds clearly 

 show the limiting effect of food on growth and fecundity. 



A number of biological aspects of arctic Daphnia simplify the 

 analysis of growth and reproduction. First, embryonic development is 

 activated synchronously in spring, and synchrony is retained in the 

 population through much of the summer. Second, there is a single brood 

 or clutch of young carried in a brood pouch. Third, the whole population is 

 female. Fourth, clutch or brood size is proportional to length. A 

 characteristic of the Daphnia populations at Barrow is the large amount of 

 size variability, similar to that found in certain aphid populations (Taylor 

 1975). 



Field observations show year-to-year changes in both mean female 

 length and in mean number of embryos per brood. In three summers in 

 one pond (Pond C), mean brood size increased from 1 .2 to 4.2 embryos per 

 female. Mean length of the females also increased. However, the 

 regression of brood size on female length showed no significant change. 

 The major increase over the three-year period was in size of the mothers, 

 hence in a larger fecundity per female. A simulation model indicates that 

 an increased food supply or a larger size at hatching allowed the females to 

 grow to a larger size. That is, the larger individuals acquire more food and 

 grow to a larger size, hence produce more young. (In 1977 the length and 

 brood size were once again at the 1971 values in Pond C.) 



Support for the function of initial size on adult length was provided by 

 SUNDAY, the simulation model (Stross et al. 1979). A regression of 

 brood size on maternal length was made to simulate the data observed in 

 field populations. In the simulation larger adults and a larger brood were 

 obtainable either by increasing the food concentration or by increasing the 

 initial size of the newborn (a sample output is shown in Figure 6-13). 



The sensitivity of the SUNDAY model to environment is illustrated 

 with two sample problems. In the first problem the model was asked to 

 determine the effect of shifting the phase of the endogenous oscillation in 

 metabolic activity. Incorporated into the model was the knowledge that a 

 feeding activity may be maximum twice each day in Pond C and also that 

 maximum activity is twice the minimum (Chisholm et al. 1975). A daily 

 temperature oscillation was employed that corresponds to observed values 

 on clear summer days (5° to 15°C). 



The strategy of field populations of Daphnia was thereby suggested. 

 As a result of having two daily peaks of activity each day with a maximum 



