Zooplankton 295 



Because there are no fish in the ponds, most of the predation on 

 crustaceans is by other zooplankton. In artificial experiments in small 

 containers, an adult or copepodid stage of Heterocope killed young 

 Daphnia with a daily coefficient of 0.14 liter ' The coefficient for adult 

 Daphnia was 0.09. Heterocope also feed on Cyclops nauplii while adult 

 Cyclops feed on their own nauplii, on young Heterocope, and on 

 fairyshrimp. In the field, the Daphnia which first hatch from the 

 overwintering eggs are relatively immune from predation because the 

 Cyclops adults are small and because the Heterocope are still in their 

 naupliar stage. However, by mid-July when the single brood of Daphnia 

 hatches, the Heterocope are adult and appear to eat nearly the entire 

 brood. It is possible that the large size of Daphnia and other crustaceans is 

 an adaptation to minimize vulnerability to invertebrate predation. 

 Another possibility is that the large size results from competition for food. 



The production of zooplankton was calculated as the biomass of 

 individuals surviving to the end of the summer plus the loss from mortality 

 plus the mass of eggs produced. The annual production was close to 1 mg 

 C liter' or 200 mg C m ^ In one pond, the fairyshrimp contributed 0.8, 

 0.1, and 0.1 mg C in each of three separate years while Daphnia 

 contributed 0.2, 0.8, and 0.5 mg C in the same years. 



Daphnia filtration rate was at a maximum at 12°C, much lower than 

 the maximum temperature for temperate species. When compared with 

 individuals of the same size of other species of Daphnia, D. 

 middendorffiana filters slightly faster. A 2.6-mm-long animal has a 

 filtration rate of 8.0 ml hr ' at the saturation level of 37,000 particles 

 ml ' (the experiment was run at 11°C). In the pond, the Daphnia are 

 believed to feed at or near their maximum rate. 



Because the relationship between size of animal and filtering rate is a 

 power function, the velocities increase rapidly as the animals grow. If an 

 efficiency of assimilation of 70% is assumed (no measurements were 

 made), then the growth rate of animals can be related to food 

 concentration and animal size. Thus, a 1 .5-mm animal can barely grow at 

 50 Mg POC liter " ' while a 3.0-mm animal will not only grow well but will 

 also produce 10 eggs. Thus, large animals have a decided advantage over 

 small animals at all stages of growth and only large animals can hope to 

 accumulate enough stored energy to reproduce within the short summer. 

 Animals in the ponds follow this general rule too; the reproductive 

 potential of 3.0-mm Daphnia wsls 4 to 5 times that of a 2. 5-mm Daphnia. 



In the ponds, the zooplankton have the additional problem of coping 

 with large quantities of non-living particles. Some of these may well be 

 edible but most are likely resistant to breakdown. Thus, there was about 

 100 Mg living C liter ' and 250-800 ^g non-living C liter ' in a typical 

 pond. Nothing is known of the effect of these non-living particles on 

 Daphnia; at one extreme, they might digest some or most of them while at 



