Zooplankton 267 



Chisholm et al. 1975, for details). Incubation times were kept short so that 

 there was no loss of label from defecation or excretion. The labeled algae 

 and Daphnia were added to pond water which had been filtered through a 

 64-^m mesh net. Final numbers of particles in the incubation medium 

 were determined with a Coulter Model B Electronic Cell Counter. The 

 amount of radioactivity both added and taken up was measured in a liquid 

 scintillation counter after digestion of the organisms with Protosol and 

 maceration with a tissue homogenizer. 



In D. middendorffiana, the relationships of the feeding rate to food 

 concentration, to body length, and to temperature showed generally the 

 same pattern found for other Daphnia species. Maximum filtering rate for 

 all size classes of Z). middendorffiana at Barrow was observed at 12°C. In 

 other species of Daphnia, the maximum filtering rate was reported at 

 higher temperatures (McMahon and Rigler 1965, Burns and Rigler 1967). 



Before the experiments, the freshly caught animals were held 

 overnight in constant dim light. Aside from this, no attempt was made to 

 acclimate the animals because the test temperatures (roughly 5°, 10°, 

 15°C) were within the range of the natural temperature changes on clear 

 summer days. Yet acclimation may be important for animals grown at a 

 constant temperature because one species, D. rosea, has a temperature 

 optimum for filtering rate that was entirely a function of the temperature 

 (12° to20°C) at which the animals were grown (Kibby 1971). 



The filtering rate of D. middendorffiana is slightly higher than rates 

 for other species of Daphnia. The relationship of this rate, F (ml per 

 animal hr '), to body length L (mm), can be expressed as a power 



I ^ 



c 

 o 



6--. 



o 

 cr 



c 4 



-7- 2 



50 



10 20 30 40 50 60 



Food Density, lO' cells mf' 



FIGURE 6-9. Filtering and feeding rates for individu- 

 als of Daphnia middendorffiana as determined by 

 feeding natural suspensions of particulates containing 

 labeled cells of Chlamydomonas reinhardtii. 



