Quantitative assessment of trophic flow based on polyunsaturated fatty acids reveals that high‐quality algae support aquatic food webs irrespective of dams

Abstract High‐quality, yet less abundant, food sources sustain river food webs. However, the impacts of dams on these sources and their nutritional contributions to aquatic animals are not well understood. A substantial knowledge gap exists in understanding the quantitative contribution of basal food sources, such as periphyton, to aquatic animal diets and their role in supporting aquatic animals. This study fills this gap by using long‐chain polyunsaturated fatty acids as biomarkers to quantify the dietary contributions of periphyton, submerged leaves, and macrophytes to macroinvertebrates and fish in dammed rivers. Our results showed that periphyton contributed more to the lipid diets of macroinvertebrates and fish than submerged leaves and macrophytes, regardless of dams, and was a high‐quality food source supporting aquatic food webs. The dietary contribution of periphyton to macroinvertebrates decreased at the downstream sites of small dams compared to the upstream sites, while the contribution of leaves and macrophytes increased due to dam‐induced changes in periphyton nutritional quality. The nutritional quality of periphyton correlated positively with its contribution to invertebrate grazers and filterers, while its contribution to fish varied depending on the feeding strategies of fish species. Our findings underscore periphyton as a high‐quality dietary source in dammed river ecosystems and suggest that improving its nutritional quality enhances its contribution to the lipid diets of macroinvertebrates and fish.