276 R. G. Stress et al. 



where L is length in mm. In D. pulex the exponent b is between 2.6 and 

 2.8 (Burns 1969) while in D. middendorffiana the exponent is 

 approximately 3.0 at the optimum temperature; this is suggestive of a 

 regression with a steeper slope (Chisholm et al. 1975). Other species, such 

 as D. rosea, are also likely to show a steeper regression of filtering rate on 

 body length (Burns and Rigler 1967). 



The steep regression for the relative increase of filtering in D. 

 middendorffiana is matched by a steep regression of brood size on 

 maternal length. Brood size in Daphnia is closely correlated with maternal 

 length (Green 1954, Hall 1964, Stross and Chisholm 1975). There is a 

 faster relative increase of fecundity in D. middendorffiana, the species 

 from the more oligotrophic polygon ponds. Both feeding and reproductive 

 adjustments in D. middendorffiana are consistent with existence in sparse 

 food environments. 



The life cycle of Daphnia could also be a significant contributor to its 

 dominance over the fairyshrimp species. The Daphnia, unlike all other 

 Crustacea in the plankton, have a second generation each year (Figure 6- 

 12). The single brood of young which is produced by the overwintering 

 generation could permit a feeding advantage over other species of 

 Crustacea. The mechanism is unknown, however. The alternative 

 argument, that invertebrate predation forces the allopatry of these two 

 species of Daphnia, has been advanced by Dodson (1975). 



Control of Number of Generations 



Annual production rate can be directly proportional to the number of 

 generations when rates of birth and survival are constant. An experimental 

 basis for the determination of the control of the number of generations of 

 Daphnia at Barrow each year has been provided (Stross 1969). The effect 

 of a restricted number of generations on productivity has already been 

 discussed (Stross and Kangas 1969). A primary question is whether the 

 environment or some intrinsic quality of the species regulates the number 

 of generations in the planktonic Crustacea. 



The fairyshrimps are widely known to be restricted to a single 

 generation, with an obligatory arrest appearing in the embryos (Broch 

 1965). Life cycle control in the copepods at Barrow is uninvestigated, 

 although it is likely that cyclopoid and calanoid species may function 

 differently. The cyclopoids enter diapause in late juvenile (copepodid) 

 stages and do so near the summer solstice while the sun is continuously 

 above the horizon. The species of Cyclops in the ponds at Barrow could be 

 long-day inductive if they function as suggested by the work of Spindler 

 (1971a, 1971b). 



Whether control is obligatory or facultative, the fairyshrimps and 

 copepods have but a single generation each year. Thus, while the ice-free 

 season is admittedly short the populations fail even to fill that time interval 



