Periphyton as alternative food source for the filter-feeding cladoceran Daphnia magna

1.  Daphnia magna , a well-studied primary consumer, is mainly known as a filter feeder. In this study, we investigated the ability of D. magna to use periphyton as an alternative food source to phytoplankton. We examined the development of laboratory populations fed with different food sources ( Desmodesmus subspicatus and/or periphyton or neither) over a period of 42 days, and observed the behaviour of the daphnids.
2.  The addition of periphyton to phytoplankton food led to an increase of daphnid population biomass. When fed with periphyton as the only food source, a small but stable D. magna population developed.
3.  The behaviour of daphnids fed with both food sources revealed a preference for feeding on D. subspicatus . Only below a concentration of D. subspicatus of approximately 0.05 mg C L^?1 (0.4 × 10⁷ cells L^?1) did D. magna use periphyton as an alternative food source.
4.  Periphyton showed distinct reactions to grazing by D. magna . The thickness of the periphyton layer was reduced from about 4 to 1 mm and we observed a change in species composition due to grazing.
5.  The ability of D. magna to graze on periphyton could serve to stabilize its population density and reinforce its competitive advantage over other cladocerans. By switching between food sources, D. magna can act as a coupler between pelagic and benthic habitats and food webs.