254 



R. G. Stress et al. 



some sort of physiological "dark interval." Other arthropods have a 

 similar interval timer that determines when they can respond to a 

 photoperiodic induction (Lees 1960); in the Daphnia, the production of a 

 brood of young may also be an interval timer. 



From the foregoing account it is clear that the initial density of 

 zooplankton populations in spring is determined by the number of 

 overwintering animals that emerge. An upper limit to density is thus 

 placed on all monovoltine species and therefore on the annual production. 

 The Daphnia have an additional production capability from the young 

 produced by the overwintering generation. 



Standing Crops 



The Crustacea were collected by pouring 8, 10, or 12 liters of water 

 through a No. 25 mesh plankton net. The net could not be towed in the 

 pond because it would have disturbed the sediment; instead, 1- or 3-liter 

 water samples were dipped from the pond with a plastic bucket mounted 

 on a long pole. This procedure may have resulted in underestimates of the 

 fast moving animals such as the calanoid copepods and of the bottom 

 buggers such as the fairyshrimps. Triplicate samples were taken at one or 

 more stations, depending on the pond being sampled. 



The distribution of the zooplankton within a pond was not random. 

 Fairyshrimps and Daphnia were frequently clumped, either as a dense 

 swarm {Daphnia) or as a reproductive group (fairyshrimps). The two 

 species of fairyshrimps were segregated one from the other in the mating 

 swarms and the individuals or attached pairs often hovered in rows. 

 Obviously, there was poor sampling precision for these forms. The 



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FIGURE 6-2. Densities of various stages of Cyclops 

 vernalis in Pond C, 1971. Sample points are means of 

 four to eight 10-liter samples. 



