Zooplankton 277 



with reproduction. Surely there can be no direct connection between 

 available length of the season at Barrow and productivity in tundra ponds. 



In the case of the two species of pond Daphnia the number of 

 generations each year is known to be under the influence of environment 

 (Stross 1969). In the laboratory multiple broods of young are produced at 

 constant temperature and long-day photoperiods. In the field the first 

 generation overwinters as embryos (ephippial eggs) (Edmondson 1955, 

 Stross and Kangas 1969). The overwintering generation produces but one 

 brood of young. It then produces the resting or ephippial eggs for the 

 remainder of the season. Two or three broods of resting embryos are likely 

 if the adult survives. The phenological pattern applies to both species of 

 Daphnia in Barrow ponds. 



In the laboratory the overwintering generation of D. pulex was shown 

 to be capable of producing broods of young throughout life (Stross 1969). 

 The controlling stimulus at constant temperature was day length. At a 

 photoperiod of L22:D2, 50% of the broods were diapausing; this is the 

 longest critical photoperiod on record (Stross 1969). The switch in the field 

 was postulated to involve temperature but as a token stimulus that 

 interacted with photoperiod. Later experiments with D. middendorffiana 

 with temperature held constant under a field (natural) light regimen have 

 shown that a temperature of 10°C allows successive broods of young to be 

 produced. 



In other words, low summer temperatures in ponds at Barrow may 

 not limit production directly. The temperature, particularly the oscillating 

 temperatures, acts in conjunction with photoperiod to regulate the 

 reproductive polymorphism of the dominant species. The presence of 

 sufficient degree-days to allow for more broods of young has been 

 discussed (Stross and Kangas 1969). The question is whether the switching 

 mechanism that determines broods of young vs. broods of resting embryos 

 can be modified to allow more than one brood of young. 



The first brood of young Daphnia cannot be stopped. It is the result of 

 an internal switching mechanism analogous to that in aphids (Lees 1965). 

 It prevents the environment from triggering the transformation until after 

 one brood of young is released. However, the reproductive pattern in 

 Daphnia at Barrow could represent an adaptation. Olofsson (1918) 

 reported populations of Daphnia on Spitzbergen (Svalbard) that have one, 

 two, or no broods of young before starting to release diapausing embryos. 

 However, Meijering (personal communication) has found that the annual 

 cycle on Svalbard is the same as that at Barrow. 



Reproductive Effort 



The realization is that annual production is limited by the density and 

 fecundity of the overwintering generation. The studies described in the 

 previous section show that reproductive effort in both Daphnia and the 



