Zooplankton 279 



2.0 24 2.8 



Malernal Length, mm 



FIGURE 6-13. Simulation of the number of embryos 

 in the brood pouch ofinstar 6 as a function of length 

 of the female in instar 5 of a Daphnia population 

 (solid line). Simulation is from a sunda y Daphnia 

 population fed at various concentrations of food and 

 beginning at various initial lengths. Note the biphasic 

 slope which flexes at the length where growth effici- 

 ency reaches its maximum. The regressions for two 

 real populations from arctic pools are shown for 

 comparison (dashed line). The left is from D. pulex, 

 the right from D. middendorffiana, which is also bi- 

 phasic under experimental conditions. (After Stross 

 et al. 1979.) 



activity near noon and again at midnight, the reproductive rate was 

 doubled. There was only a small increase in length of the female (Figure 6- 

 14). Haney and Buchanan (personal communication) in direct field 

 measurements have shown a daily oscillation in feeding activity that is out 

 of phase with temperature. They found only one maximum each day, 

 although their measurements were in different ponds from those of 

 Chisholmetal.(1975). 



A part of the increased reproductive effort was attributed to a faster 

 rate of food ingestion and a slower rate of respiration. The so-called 

 McLaren (1963, 1974) effect was tested in a second modeling exercise. 

 When night temperatures were set at a constant 14° or 15°C and prevented 

 from dropping, growth was much reduced and the individuals failed to 



