Food consumers in natural environments often face challenges in meeting their exact nutritional demands, as available food resources can vary significantly in quality, being classified as either good-quality or poor-quality. In aquatic ecosystems, the introduction of poor-quality food alongside existing good-quality resources is a common phenomenon. Examples include the influx of terrestrial organic matter to aquatic ecosystems or the occurrence of cyanobacterial blooms. Previous studies have extensively explored how consumers adapt to limited phosphorus availability by regulating carbon pathways—such as egestion, excretion, respiration, and growth—to eliminate excess carbon. However, consumers fulfill their elemental requirement only by ingesting and metabolizing organic carbon, which are less studied. Using carbon stable isotope labeling, this study investigated the detailed carbon pathways of Daphnia magna fed a diet of insufficient Scenedesmus bijuga (13C-labeled) supplemented with increased concentrations of poor-quality Microcystis wesenbergii (non-toxic strains). The results revealed an adaptive strategy employed by consumers: surplus feeding, coupled with enhanced consumption of Microcystis-derived organic carbon in excretion, and respiration, so as to spare more Scenedesmus-derived organic carbon for growth. And the strategy was particularly pronounced under conditions of more severe food quality constraints but higher food quantity. The study highlights the subsidy effect of poor-quality organic carbon, which may provides additional energetic and micro-molecular benefits to consumers while allowing them to conserve more high-quality organic carbon in their biomass for physiological needs. These findings offer valuable insights into the biological mechanisms underlying consumer stoichiometry, enhancing our understanding of how organisms balance their biochemical demands in dynamic and resource-limited environments.