Zooplankton 275 



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JUNE 



FIGURE 6-12. A model of production in an arctic consumer and its con- 

 trol. The model attempts to summarize the components of production in 

 a dominant species, such as Daphnia middendorffiana, and to illustrate 

 how production may be restricted by a variety of agents within the organ- 

 ism, or population, and by the external environment. A — Time limita- 

 tion placed on the interval of potential productivity by temperature. The 

 interval is the ice-free interval from mid- June to September. B — The sea- 

 sonal cycle in Daphnia, starting with the hatching of the overwintering 

 embryos (B-1). After one brood of young (B-2), reproduction switches to 

 the eggs that enter a diapause as embryos (B-3). C — The switch in 

 Daphnia reproduction which limits the number of young produced to the 

 equivalent of one brood is controlled intrinsically by the environment. 

 D — Cumulative yield based on each source of product, namely the over- 

 wintering and the young-of-year generation. Two major forces are at 

 work in determining cumulative yield. In a mortality-free environment 

 (D-1) actual yield seems to be determined by the biomass present. That 

 is, the larger the population the larger the yield. Explanations are provid- 

 ed in the text. In actuality, mortality (D-2) reduces the potential yield. 

 See text for further discussion. 



