Zooplankton 263 



already mentioned, only 35 out of 127 eggs produced per liter gave rise to 

 nauplii, and the first hatch was much more successful than later hatches. 



Mortality rates of fairyshrimps and Daphnia were high in 1971. In 

 fact, the loss of young fairyshrimp was so high early in the season that it 

 resembled an exponential function (Figure 6-4). The loss o{ Daphnia was 

 always largest in the young-of-the-year generation where 85 to 90% of the 

 newborn did not survive. Much of this loss may be the result of predation 

 by Heterocope, the large copepod. 



We conclude that most of the loss from the populations occurred in 

 the young zooplankton stages. The adults all die at or before freezeup but 

 the exact time is unknown. The percentage survival of the overwintering 

 embryos is also unknown. 



Predation 



There are no fish and the feeding of phalaropes is not important; 

 therefore, most of the predation on Crustacea is by other Crustacea. A 

 number of experiments were carried out by placing various numbers of 

 prey in small jars along with a fixed number of predators. The jars were 

 incubated on the bottom of a pond for a number of days. The data give 

 possible relationships among these animals but can not give quantitative 

 relationships because the conditions are artificial. For example, Strickler 

 and Bal (1973) have shown that some copepods find their prey by 

 following turbulent wakes. Placing the animals inside a bottle may 

 drastically change the natural turbulence and may well give predators an 

 advantage. A measure of the possible intensity of predation is the 

 coefficient, A^ (liters time ~ '), calculated from 



Prey remaining = (Prey available at To)e 



-KTX 



where Tis time and X is number of predators liter~\ 



From 16 to 21 July 1972, young Daphnia were eaten by Heterocope 

 (five stage V copepodids or adults liter"') with K of 0.69 liter — this is a 

 daily K of 0.14. Adult Daphnia were killed at a daily K of 0.09 liter. From 

 23 to 31 July, both young Daphnia (slightly larger now) and fairyshrimp 

 were killed at a daily K of 0.09. 



Experiments by Dodson (1975) in 1973 tested more of the possible 

 feeding relationships within the food web (Figure 6-7). These experiments 

 were run in 220 ml of filtered (93-Mm mesh net) pond water for 48 hours. 

 Data for all copepod nauplii were combined so the nauplii of Cyclops 

 vernalis also includes those of C. strenuus, C. languidoides, and 

 Diaptomus alaskaensis. 



The value of K for the interactions (Table 6-6) confirms the voracity 

 of Heterocope for Daphnia young and indicates that they feed equally well 

 on Cyclops nauplii. Adult Cyclops vernalis had a high K for their own 



