262 R. G. Stress et al. 



TABLE 6-5 Mean Weights of Eggs and of the Maximum Size of Adults, and 

 Specific Growth Rates (g day~^ ) for Crustacea in the Plankton 

 of an Arctic Pond ^ 



Daphnia middendorffiana 



Polyartemiella 



Branchinecta 



Heterocope 



Diaptomus glacialis 



D. bacillifer 



Cyclops strenuus 



C. vernalis 



C.magnuus 0.40 0.20 21.8 25.0 6/15 to 7/5 



a. Calculated refers to the value from regressions based on measured carbon content; 

 observed values are means of field data. 



b. Animals hatched from overwintering embryos. 



c. Young-of-the-year animals. 



A mean of 3.7 ^g C per egg was determined for the non-diapausing D. 

 middendorffiana; this was similar to the weight of the diapausing eggs. 

 There was no weight difference between the two kinds of eggs of D. magna 

 (Green 1956). The mean size of the Z)a/?/?ma was 41 ^g C (range was 33 to 

 70 Mg). For our calculations, we assumed that the overwintering 

 generation of Daphnia stopped growing in mid-July when the biomass 

 seems to approach an equilibrium (Winberg 1971) (Figure 6-5), even 

 though this species is indeterminate in growth. The fairyshrimp 

 Polyartemiella averaged 416 Mg C per animal and Branchinecta 307 Mg C. 



Growth Rates 



Coefficients of growth and maximum size attained by each of the 

 major sjbecies were calculated from regressions. For convenience, the 

 growth of all species of Crustacea was assumed to have both a predictable 

 interval and an upper limit (Table 6-5). Daily rates of instantaneous 

 growth were estimated to range from 0.09 for Heterocope to 0.20 for the 

 cyclopoid copepods. 



Mortality 



Estimates of mortality were made from data on population size over 

 time. The largest loss in 1971 was of the Cyclops embryos or nauplii. As 



