IMPACTS OF PREY PREFERENCE BY DAPHNIA PULEX ON ALGAL PRODUCTIVITY

Laboratory experiments estimated the impact of different Daphnia pulex densities on chlorophyll a and phaeopigment concentrations, and primary productivity. Chlorophyll a declined with increasing Daphnia density, while phaeopigment concentrations increased linearly with increasing Zooplankton density. These results are consistent with theoretical predictions regarding the importance of predator-prey interactions in aquatic food webs. Additional studies employed Carbon-14 labelling techniques to determine feeding preferences of Daphnia on cultures of various combinations of Chlamydomonas and Chlorella. In these studies a negative relationship occurred between grazing pressure and primary productivity, whereas food web theory predicts a unimodal relationship. Labelling experiments showed a lack of preferential feeding in Daphnia for either phytoplankton species. The implications of these results to theories of optimal foraging and cascading food web interactions are examined.