Biotic and abiotic controls of food chain length and trophic structure in two contrasting River Basins of China

Food chain length (FCL) and trophic structure are key attributes of aquatic ecosystems, yet their responses to biotic and abiotic conditions may vary among river systems under different climatic and land-use settings. In this study, we compared trophic metrics between the temperate Taizi River (TR) and the subtropical Gan River (GR) in China and examined how land use, water quality, and aquatic biodiversity were associated with food-web structure. Four trophic metrics—maximum trophic level (MaxTL), mean trophic level (MeanTL), trophic level range (RanTL), and trophic level variance (VarTL)—were calculated for fish assemblages. Their relationships with environmental variables were analyzed using redundancy analysis (RDA) and structural equation modeling (SEM). The GR exhibited significantly higher MaxTL, RanTL, and VarTL than the TR, indicating greater trophic breadth and complexity in the subtropical basin. RDA showed that the environmental correlates of trophic metrics differed between basins: in the TR, MaxTL and MeanTL were more closely associated with algal diversity and forest cover, whereas in the GR they were more strongly related to organic pollution and forest cover. SEM further suggested that the pathways linking land use, water quality, biodiversity, and trophic metrics were not identical between the two rivers, although these pathway results should be interpreted cautiously because model fit differed between basins. Overall, our findings indicate that trophic structure in river ecosystems is context dependent and shaped by basin-specific combinations of biotic and abiotic factors. This study provides a comparative perspective on food-web variation between climatically contrasting rivers and offers implications for basin-specific ecological management.