Ecological stoichiometry characteristics and influencing factors of the source reservoir in the middle route of the South-to-North Water Diversion Project.

To explore the interaction between aquatic organisms and the environment in the Danjiangkou Reservoir, the source reservoir for the Middle Route of China's South-to-North Water Diversion Project, this study analyzed the ecological stoichiometric characteristics of carbon, nitrogen, and phosphorus in main species and their influencing factors. The results revealed that there were no significant differences in the contents of nitrogen and phosphorus in fish between the upstream and downstream areas. Compared to omnivorous and filter-feeding species, carnivorous fish in both the upstream and downstream areas displayed elevated nitrogen and phosphorus contents alongside reduced carbon content. The phytoplankton and aquatic plants showed significant differences in nitrogen content between areas(P < 0.05), but no significant differences were observed in the nitrogen and phosphorus contents of other biological groups between the upstream and downstream reservoir areas. The nitrogen and phosphorus stoichiometric characteristics of biological components in the Danjiangkou Reservoir showed strong homeostasis. A further correlation analysis was conducted between the nitrogen and phosphorus stoichiometric characteristics of fish and water quality including TN, NH₄⁺-N, NO₃⁻-N, NO₂⁻-N, and TP. The results showed that there was no correlation between the nitrogen and phosphorus stoichiometric characteristics of fish and water quality in the upstream reservoir area. In the downstream reservoir area, the N: P in fish was significantly positively correlated with NO₂⁻-N (P < 0.05) and extremely significantly negatively correlated with NH₄⁺-N (P < 0.01). The study demonstrated that the fish stoichiometry in large deep-water reservoirs was primarily shaped by species identity and feeding ecology, with limited influence from spatial heterogeneity in water quality. These findings provided critical insights into nutrient cycling and ecosystem stability, supporting sustainable management of this vital water diversion source.