256 R. G. Stress et al. 



- 1.6 



o 

 •a 

 1 1.2 



a. 



0.8 



■5 0.4 



2.4 1 r 



2.0 



1973 



Bronchinecta 



and 

 PolyartemiellQ 



_i L 



20 



Jun 



4- 



10 20 



Jul 



10 

 Aug 



20 



FIGURE 6-4. Densities of fairyshrimps Bran- 

 chinecta paludosa and Polyartemiella hazeni in 

 Pond C, 1971. (Same samples as Figure 6-2.) 



The calanoid copepods overwinter as embryos. They hatched within a 

 week of the pond thaw (Figure 6-3) and by 19 June there were two sizes of 

 calanoid nauplii present. The most abundant species in the study ponds 

 was Diaptomus bacillifer at about 35 liter '.It was also the smallest. The 

 medium-sized D. glacialis had a density of 8 liter ' while the largest 

 calanoid, the predaceous Heterocope septentnonalis, was not detected 

 until 3 July when 2.5 liter' were recorded. Densities of all populations 

 decreased as the individuals developed, although after 15 July numbers 

 stabilized at 3 liter"' for D. bacillifer, less than 1 liter ' for D. glacialis, 

 and about 2 liter " ' for Heterocope. 



The fairyshrimp are very large in comparison with the other 

 zooplankton. For example, their mass may be 10 times that of a large 

 daphnid. As a result, they were the dominant species in biomass in 1971 

 even though the numbers were low (Figure 6-4). The two species, 

 Branchinecta and Polyartemiella, hatched at about the same time as the 

 cyclopoid copepods emerged and then reached a maximum density of 2.3 

 nauplii liter' on 21 June. The fairyshrimp numbers then declined to 

 about 0.3 liter"' in early July and by early August they were too rare to 

 appear in the samples. During mating, which took place in the 3rd week in 

 July, the fairyshrimp of each species moved to one place in the pond, 

 formed pairs, and hovered in large rafts immediately beneath the surface. 

 The fairyshrimp numbers declined throughout the 3 years of the study; in 



