Zooplankton 269 



function equation, F=0.458 I'" at 11°C and F= 0.458 /.'"' at 5°C 

 (Chisholm et al. 1975 and Table 6-8). Because both the coefficient and the 

 exponents are higher than those for other species, an individual D. 

 middendorffiana will filter more water than individuals of other species of 

 the same size. 



The feeding rates increased hyperbolically with increasing food 

 concentration (Figure 6-9) and reached saturation at a food concentration 

 of 37,000 particles ml ' ' and a filtration rate of 8.0 ml hr ^ (for a 2.6-mm 

 animal). This saturated feeding rate of the adults decreased considerably 

 over the summer; at 1 1°C on 5 July the rate was 265,000±76,000. Since 

 these rates were measured with the '^C-labeled Chlamydomonas plus the 

 natural assemblages of particles, and thus included a wide range of sizes, 

 the saturation values can not be compared with other measurements where 

 monospecific suspensions were used. 



The laboratory feeding measurements were carried out with the 

 natural assemblage of particles less than 64 ^m still present. This is closer 

 to natural conditions than most other experiments where pure cultures of 

 algae are used, but still leaves the problem of how to extrapolate the 

 laboratory data back to field conditions. One approach is to measure the 

 amount of the particulate organic carbon (POC) both in the pond and in 

 the experiments. We assumed that the 10,000 particles ml ' measured in 

 the experiments were spheres with an average diameter of 10 ^im. From 

 the usual assumptions about phytoplankton density and carbon content, 

 we calculated that 10,000 particles contained 0.2 Mg C. In the field, the 

 POC ranged from 0.2 to 0.8 Mg C ml ', so it is reasonable that the 

 Daphnia in the pond were always feeding at or near their maximum rate 

 (Figure 6-9). Additional evidence that the experimentally measured 

 filtering rates simulate nature comes from measurements by J. Haney 

 (personal communication) using ^^P-labeled yeast cells (3 ^im) added to 

 unfiltered pond water. The results agreed with the estimates given above 

 and in Table 6-8. 



If we accept the filtering rates above as close to natural rates, then the 

 effect of food density, temperature, and length of animal on growth and 

 reproduction could be calculated if the assimilation efficiency were known. 

 Literature estimates of assimilation efficiency vary widely and we made no 

 independent measurements. Our arbitrary assumption of 70% efficiency is 

 useful for illustrating the various interactions, but the results of the 

 calculations (Table 6-8) have only comparative value. In the table, the 

 ingestion rate per individual equaled 2.24 L^ where L is length (mm). The 

 respiration rate was a mean of our measurements in a Gilson 

 Respirometer (4.0 to 8.0 m1 Oi (mg C) ' hr"' at 10°C). These rates agree 

 in general with those measured by other investigators although our 

 precision was not as good as hoped for. 



From these crude calculations (Table 6-8), we estimate that the 

 smallest animal (1.5 mm) can barely grow at a food density of 50 ^g C 

 liter '. At 60 Mg C the growth rate is doubled but still is very low. At 100 

 Mg C liter ' the individual may grow at a rate comparable to that actually 

 observed at Barrow (Table 6-5). 



