The growth/reproduction/physiological rates/stable isotopic values of Daphnia with different diets

1. Food consumers in natural environments often face challenges in meeting their exact nutritional demands, as available food resources can vary significantly in quality. In aquatic ecosystems, the introduction of poor-quality food alongside already insufficient good-quality resources is a common phenomenon – for example, through terrestrial subsidies or the occurrence of cyanobacterial blooms. 2. Consumers may metabolize organic carbon of differing quality in distinct ways to better conserved essential biochemicals and enhance performance. However, studies in this topic are scarce. 3. Using carbon stable isotope labeling, we investigated the detailed carbon pathways in Daphnia magna fed a diet of Scenedesmus bijuga (13C-labeled, good-quality organic carbon) insufficient amounts for growth supplemented with increasing concentrations of poor-quality Microcystis wesenbergii (non-toxic strains, poor-quality organic carbon). 4. The results revealed an adaptive strategy employed by consumers: surplus feeding, coupled with enhanced consumption of Microcystis-derived organic carbon in excretion, and respiration, allowing retention of good-quality organic carbon for growth. This strategy was particularly pronounced under more severe food quality constraints but higher food quantity.