274 R.G. Stress etal. 



under natural conditions, nearly half of the animals molted within 1 hour 

 around 0100. The second monitor also showed a similar burst of molting 

 but it occurred 12 hours later (1300). 



CONTROL OF ZOOPLANKTON PRODUCTION (I)* 



General Considerations 



Populations of Crustacea in arctic pond ecosystems exemplify the 

 antithesis of the steady state. Each population spends only a small part of 

 the year in the active phase of the life cycle and the remainder of the year 

 in diapause. The annual cycles are mostly monovoltine. The switch from 

 diapause to active phase each spring is nearly synchronous. Stable age 

 distributions, demanded of populations in the steady state, are therefore 

 not possible. The populations may be viewed as batch cultures (Smith 

 1952, 1963) in which endogenous constraints of the genotype rather than 

 resources prevent attainment of the steady state. 



An analysis of production and its control was carried out under the 

 principles set forth by Slobodkin (1959, 1960). The biotope is viewed 

 through the eyes of the dominant planktonic consumer, Daphnia 

 middendorffiana. The questions seemingly most pertinent to both the 

 answers available and the uniqueness of the arctic community are: (1) How 

 are Daphma populations able to dominate the assemblage? (2) What 

 determines the number of generations each year? (3) What controls 

 reproductive effort of each generation? (4) What, if anything, prevents the 

 maximum yield from each generation? 



Polygon ponds near Barrow and elsewhere in arctic Alaska are 

 populated with Crustacea that feed by filtering particulates from the 

 water. The large size of both the dominant species, D. middendorffiana, 

 and the fairyshrimps, in combination with the absence of vertebrate 

 predators, suggests that large size could be an adjustment in the face of 

 competition (Hrba^ek 1962, Brooks and Dodson 1965). An alternative 

 argument presented by Dodson (1972, 1974) attributes the large size to the 

 selective action of invertebrate predators. 



A second type of small pond at Barrow, the trough pond, contains 

 Daphnia pulex. Fairyshrimps are present but exceedingly rare. D. pulex is 

 absent from polygon ponds, presumably as a result of inferior competitive 

 ability or as a result of greater vulnerability to predators. This species may 

 be less able to satisfy its food needs at densities of pond particles found in 

 the polygon ponds. Both the filtering power and its relative growth with 

 body size are significantly smaller in D. pulex. Filtering rate F equals aL 



*R. Stress 



