We investigated the ability of the water flea, Daphnia, to adapt the size and structure of its filtering apparatus as a response to experimentally increased biomass of inedible filamentous and colonial cyanobacteria in a large in situ enclosure experiment. Predator-induced phenotypic plasticity in Daphnia has been extensively documented, but only a small number of studies have focused on morphological changes induced by food quantity and quality. Here we show that Daphnia responded to increased biomass of inedible phytoplankton in its environment by enlarging the area and the mesh size of its filtering apparatus. These observations suggest that Daphnia responds to increased concentrations of inedible particles in the same fashion as it does in a very low food environment. In our study, daphnids exposed to a high biomass of inedible algae, in fertilized enclosures, had significantly larger (12-15%) filter screens attached to their third and fourth limbs in comparison to daphnids exposed to a low biomass of inedible algae. The mesh size also increased in the same conditions. These results suggest that daphnids used their phenotypic plasticity to respond to changes in their food quality and quantity. By using this strategy, daphnids can maximize their food uptake and hence compensate for the scarcity of suitable food encountered in very oligotrophic conditions or even in eutrophic conditions when phytoplankton communities are dominated by large inedible species.