<p>Understanding trophic relationships is fundamental for investigating ecosystem processes and their role in community dynamics. Several theories on trophic organisation of biotic communities have explored how resource use, niche breadth, and trophic position vary among organisms. Traditional approaches typically assume invariant trophic traits within species, whereas alternative theories emphasize size-structured relationships, where trophic attributes scale with body size across taxonomic groups – often leading to controversial interpretations. In this study, we examined the relative roles of species identity and body size in the trophic organisation of a large shallow lake fish community using stable isotope analysis based on δ¹³C and δ¹⁵N. By comparing the explanatory power of alternative trophic models, we found that models incorporating both species identity and body size overperformed simpler models that included only one of these factors. Models incorporating full factorial effects of species and body size performed the best, suggesting that ontogenetic changes in resource use are species-specific, while the pure body size model provided only marginal or no explanatory power in the community level analysis. Our analysis revealed that fish in Lake Balaton rely primarily on the open-water basal carbon and nitrogen sources and most species exhibit substantial among-individual variation in both trophic position and isotopic food niche breadth. We conclude that stable isotope analysis, has the potential to provide a detailed understanding of the trophic organisation of lake fish assemblages. Based on our results, we recommend that trophic modelling of fish communities should consider both among species and size-related within species variability.</p> Graphic abstract <p></p>