294 R. G. Stress et al. 



overwinter, but in the case of the cyclopoid copepods, a copepodid stage 

 (pre-adult) enters diapause. Daphnia hatch from overwintering eggs 

 (ephippial eggs) and the hatchlings grow and produce a brood of young. 

 Next, the same animals switch to production of the ephippial eggs. It is 

 likely that the single brood of young does not survive to reproduce. Only 

 parthenogenetic females of D. middendorffiana have been found' at 

 Barrow. The switch to ephippial egg production is likely controlled both 

 by photoperiod and by temperature. 



The life cycle of the crustaceans implies that an upper limit is set on 

 the number of animals that grow in a given year by the number of 

 diapausing animals that survived the winter. 



The first animals to hatch in the spring are the cyclopoid copepods 

 {Cyclops vernalis and C. magnus). Copepodid and adult forms were 

 present almost as soon as any water at all appeared around the edge of the 

 ponds; by mid-June, egg sacs and even nauplii were present. The egg crop 

 reached its peak in early July when there were about 2.5 females, 1.7 

 males, and 65 eggs liter"'. A maximum of 35 nauplii liter ' were seen in 

 late June but in late July and in August there were only 10 animals liter 



Calanoid copepods were more abundant than the cyclopoids. Soon 

 after they hatched in mid-June there were up to 40 liter"' of Diaptomus 

 bacillifer and D. glacialis. After 15 July the numbers stabilized at about 4 

 Diaptomus liter"' and 2 Heterocope septentnonalis, a predaceous form. 

 Fairyshrimp are so large that they had the greatest quantity of biomass of 

 any zooplankton group even though their numbers were usually low. The 

 two genera, Branchinecta and Polyartemiella, hatched in mid-June and 

 soon up to 2 nauplii liter ^ ' were present. By early July there were less than 

 0.3 liter"' In some years the numbers were so low that Daphnia became 

 the dominant organism in the plankton. 



Daphnia hatch in late June and reproduce in early- to mid-July. 

 Between 2 and 5 animals liter"' are present much of the summer but only 

 about 0.5 new animals liter"' appear from the single brood. It is likely 

 that most of the young of this summer brood are eaten by Heterocope. 



In general, the zooplankton egg production was below the maximum 

 possible production for each species. For example, when grown in the 

 laboratory, the fairyshrimp produced 1.5 to 2 times more eggs than in the 

 field. Even when large numbers of eggs were produced, the resulting 

 number of nauplii or young was small. For example, Cyclops produced 

 126 eggs liter"' in one year but only 7.5 nauplii liter ' resulted. 

 Production by invertebrates is important but this can not be separated 

 from a low rate of hatching. Daphnia has the ability to produce 12 eggs per 

 brood but the average brood sizes were 2 to 4. Larger animals had larger 

 broods; this was controlled by the food supply. An experimental 3-fold 

 increase in the food concentration for Daphnia resulted in a 3-fold increase 

 in brood si2e. 



