Zooplanklon 257 



1973 there was less than 0.05 animal liter' in the intensively studied 

 ponds after the first sampling date in June. 



Young Daphnia appeared in the water several weeks after the ponds 

 melted (Figure 6-5); hatching occurred on 20 June in 1971 and on 25 and 

 28 June in 1972 and 1973. After hatching in late June, the number of 

 Daphnia should be constant or decrease until young-of-the-year begin to 

 appear in mid-July. Sampling errors cause fluctuations in the population 

 estimates, however, and in some years (e.g., 1973 in Figure 6-5) there is an 

 apparent increase. After mid-July, the young-of-the-year hatch and the 

 total numbers should increase. In 1971, the brood size was 1.2 embryos per 

 female, which should have resulted in 2.2 to 4.8 young liter " ' . Instead only 

 0.5 young liter ' were found (Figure 6-5). As discussed later, this 

 mortality is likely due to the predaceous copepod, Heterocope, and occurs 

 each year. As a result, the young-of-the-year never contribute much to the 

 total population of Daphnia. This observation leads to the conclusion that 

 the summer abundance of Daphnia is determined exclusively by the size 

 and hatching success of the overwintering population. 



Reproduction in the Crustacea 



Production of eggs by Cyclops began in mid-June. In fact, gravid 

 Cyclops vernalis were present on 14 June 1971 while there was still some 

 ice floating in the pond. 



C 



10 - 



- 8 



0) 



? 6 



a. 

 o 

 O 



- 



-1 1 '- 



(1) Initial Generation 



(2) Young - of - year 



Daphnia 



1971(2) 



_L 



20 

 Jun 



j_ 



10 20 



Jul 



10 

 Aug 



20 



FIGURE 6-5. Density of Daphnia middendorf- 

 fiana/or 1971 and 1973 of both the overwinter- 

 ing and young-of-year generations. 



