Attached algae are important primary producers in river ecosystems and are foraged by benthic macroinvertebrates and grazing fishes. The ayu Plecoglossus altivelis is an algae-feeding fish that feeds on attached algae and is an important fishery resource and conservation species in Japanese rivers. While P. altivelis uses its comb-like teeth to graze algae off the surface of stones for feeding, fine particulate organic matter (FPOM) is generated by dislodging algae into streams. Moreover, egested feces contribute to the origin of FPOM. Therefore, in this study, the contribution of feeding on algae by P. altivelis to the production of algal-derived FPOM was investigated. The produced FPOM was then evaluated to determine whether it affected the food origin of the macroinvertebrates. To investigate the population density of P. altivelis, the biomass of primary producers and food-use fraction of consumers, field surveys in the Kamo-gawa River, Japan, were conducted in the summer season from 2018 to 2020. The carbon and nitrogen stable isotope ratios of the macroinvertebrates and food sources were also measured to estimate the composition of FPOM and the contribution of food sources to primary consumers. These results showed that the algal biomass decreased at sites with high density for P. altivelis, in which the proportion of algal-derived FPOM similarly increased. Regardless of the feeding functional groups, the algal contribution of macroinvertebrates increased at sites with high algal-derived FPOM. Structural equation modeling revealed that P. altivelis feeding behavior contributed to macroinvertebrate food utilization via the production of algal-derived FPOM. These results suggested that P. altivelis is a keystone species that not only produces algal-derived FPOM but also improves the food supply to macroinvertebrates and promotes the dynamics of river ecosystems.