Attached algae are important primary producers in river ecosystems that are foraged by benthic macroinvertebrates and grazing fishes. The ayu Plecoglossus altivelis, which is an algae-feeding fish that feeds on attached algae. Fine particulate organic matter (FPOM) is generated when P. altivelis use their comb-like teeth to graze algae off the surface of stones, and dislodge them into streams. Therefore, in the present study, we investigated how P. altivelis feeding on algae contributes to the production of algae-derived FPOM. We subsequently evaluated the produced FPOM to determine whether it affected the origin of the macroinvertebrate food supply. To investigate P. altivelis density, the biomass of primary producers, and the food use fraction of consumers, field surveys were conducted in the Kamo-gawa River, Japan, in the summer season from 2018 to 2020. Carbon and nitrogen stable isotope ratios of macroinvertebrates and food sources were measured to estimate the composition of FPOM and the contribution of different food sources to primary consumers. Our results showed that algal biomass decreased at sites with a high density of P. altivelis, where the proportion of algae-derived FPOM increased. The algal contribution to macroinvertebrates increased at sites with high algae-derived FPOM. Furthermore, structural equation modelling revealed that the feeding behaviour of P. altivelis contributed to macroinvertebrate food utilisation via the production of algae-derived FPOM. These results suggest that P. altivelis is a keystone species that not only produces algae-derived FPOM but also improves the food supply to macroinvertebrates and promotes river ecosystem dynamics.