Zooplankton 253 



Polygon Pond 



Trough Pool 



Cyclops vernalis 



Cyclops magnus 



Daphnia middendorffiana 

 D. pulex 



Branchinecfa paludosa 



Polyartemiella hazeni 



Heterocope septentrionalis \ .f^^ ^^^ 



if present 



Diaptomus glacialis 



Diapfomus bacillifer 



FIGURE 6-1. Crustacea found in the plankton 

 of ponds and trough pools at Barrow, Alaska. 



in D. pulex a small number (4%) of the young-of-the-year may be infertile 

 males (Brooks 1957). 



Stross (1969) has shown that the switch from production of young to 

 production of diapausing embryos is facultative and under environmental 

 control. In an experiment with constant light and constant temperature 

 (12°C) in the laboratory, the females that hatched from the overwintering 

 eggs produced only young; at 20 hours or less of light per day only 

 diapausing embryos were produced but this switch occurred only in 

 animals which had already produced a brood of young. The critical 

 photoperiod determined in the laboratory of L22:D2 may well be acting in 

 the populations of lakes too, as these animals made the switch in late 

 August when the ratio was about L22:D2. On the other hand, populations 

 in pools and ponds make the switch in mid-July (Stross and Kangas 1969). 

 This earlier switch may be caused by the temperature regime of the ponds 

 which is warmer and more variable than that of the lakes. Thus, pond 

 temperatures may change as much as 10° in a day and may fall to 5°C on 

 any day of the summer. These low temperatures, below 10°C, may act as 



