270 



R. G. Stross et al. 



Egg production is also related to food concentration and to size of the 

 individual in much the same way as growth is related. When food is scarce, 

 only the larger individuals can accumulate sufficient energy to reproduce 

 within the time allotted. In Daphnia populations of temperate waters, the 

 time between broods is about 6 days at 10°C. In this arctic population, 6 

 days would allow the smallest individual living on 50 ^g C liter ' to eat 

 enough to produce about one-tourth of an egg. In contrast, the largest 

 animal (3.0 mm) might produce 10 eggs on the same food level. It must be 

 remembered that the Barrow Daphnia have but a single brood of young. 



One indication of the inaccuracy of the calculations in Table 6-8 is 

 that the reproductive potential of a 3.0-mm Daphnia is estimated to be 

 nearly 2 times that of a 2.5-mm Daphnia rather than the 4 to 5 times found 

 in the field. A more detailed model has been constructed which more 

 closely appro.ximates the field conditions (Stross et al. 1979). 



50 r 



« o 

 o E 



q: ■- 



o. ° 



E "^ 



« — 



to 



u 



40 



30 



20 



10- 



-k, \' 



-I — I i_ 



-J 1 I I L_ 



1600 2400 0800 1600 



a) Feeding rates at 11°C on two different days. 



I6r 



2400 0800 



Time, hr (ADT) 



1600 



b) Temperature oscillations in the pond on 

 day preceding removal of the Daphnia to 

 the laboratory. 



FIGURE 6-10. Endogenous oscillations in 

 feeding activity of D. middendorffiana in a 

 constant environment. (After Chisholm et 

 al. 1975.) 



