288 R. G. Stress et al. 



(MacArthur 1960). Factors that may control productivity of the 

 planktonic consumers are amenable to analysis. Questions regarding 

 production control, however, seem to be simpler than the questions 

 regarding density regulation. 



Productivity of zooplankton may be controlled by a combination of 

 environmental factors and intrinsic properties of organisms adapted to an 

 arctic environment. Intrinsic properties restrict the number of generations 

 each year, either with intervention from the environment, as in the case of 

 Daphnia, or without, as in the case of fairyshrimps. Thus while the pond is 

 frozen most of the year, an adapted life cycle seems not to use fully even 

 those months of a growing season that are available. 



Within the single generation, however, food resources and not 

 temperature may restrict reproductive effort. The availability of food may 

 be coupled to the density of consumers in experimental pondlets. Densities 

 are sufficiently low that a direct stimulation in growth of algae (that might 

 result from excretion) is probable. A decreased dilution effect resulting 

 from removal of detritus is an alternative possibility. 



The capacity of the consumers to stimulate the food supply makes 

 each individual vital to the effort. Young and juvenile mortality may have 

 a dual effect. Elimination of an individual before it completes growth 

 prevents its maximum yield. Its loss may also lower productivity if its 

 excretion is essential to maintain the necessary phosphate concentrations 

 for growth of algae. That is, given a single generation in which food limits 

 reproduction, the presence of an infant-killer such as Heterocope can 

 further reduce production. 



Temperature must ultimately be significant in controlling 

 productivity within arctic communities. Its immediate effect cannot be 

 discovered, however. Food resources and predation in combination with 

 intrinsic qualities of the genotype are the obvious immediate determinants. 



CONTROL OF ZOOPLANKTON PRODUCTION (II) 



Introduction 



The preceding section examined the control of zooplankton by 

 looking only at their responses, such as growth, length, and brood size. 

 When information on the composition of the food is added, then it appears 

 that this composition is exerting some amount of control on the growth 

 and reproduction of the zooplankton. From this we have formulated the 

 hypothesis that it is the relative amount of high-quality food, such as algae 

 and bacteria, in the total available food that controls zooplankton 

 production. The idea that bacteria and detritus are important to 



*M. C. Miller and R. J. Daley 



