Primary producers form the energetic base of river food webs, yet the fate of fixed carbon (C) and nitrogen (N) depends on how these nutrients are packaged within producer biomass and accessed by consumers. In nitrogen-limited rivers, biological N₂ fixation can be an important N source, but it does not uniformly support higher trophic levels. Here, we test how algal identity regulates nutrient flux and trophic transfer in the South Fork Eel River, California. Using stable isotope tracers (¹³C and ¹⁵N), we measured nutrient acquisition across common benthic producers and evaluated their contribution to the river food base by tracking assimilation by the grazing caddisfly Gumaga nigricula.
Although several algal types exhibited high rates of N₂ fixation, trophic outcomes differed sharply. Assemblages dominated by epiphytic diatoms of the genus Epithemia, which host N-fixing cyanobacterial endosymbionts, consistently supported the highest grazer assimilation and trophic transfer efficiency for both C and N. Epiphytized Cladophora and epilithic periphyton functioned as trophic highways, efficiently routing newly fixed nutrients to consumers. In contrast, free-living cyanobacteria (e.g., Nostoc and Anabaena) and mucilage-rich filamentous green algae were highly productive but transferred little of their production to grazers, forming trophic dead ends. High fixation rates alone were a poor predictor of food-web support.
These contrasting outcomes reflect differences in food quality and accessibility. Epithemia diatoms provide highly palatable, nutrient-dense biomass with favorable stoichiometry and essential biochemicals, while cyanobacteria and filamentous green algae are limited by physical structure, chemical defenses, or poor nutritional composition. This study quantifies large variation in the flow of newly fixed C and N from the atmosphere into algal biomass and onward to primary consumers, and demonstrates that trophic transfer efficiency is governed by algal functional traits. Together, these results show that shifts from diatom-dominated assemblages toward cyanobacterial or green algal dominance alter not only elemental fluxes but also the molecular pathways of nutrient assimilation, decoupling primary production from consumer support and creating trophic dead ends in river food webs.