An integrated model of eutrophication controls in reservoir-affected tributaries of the Three Gorges Reservoir

Reservoir-affected tributaries are highly susceptible to nutrient enrichment and algal proliferation, yet their trophic responses can differ markedly under broadly comparable hydrodynamic influence. Here, we examined why two adjacent 1st-order tributaries of the Three Gorges Reservoir (TGR), the Pengxi and Modao Rivers, have shown contrasting eutrophication responses despite experiencing broadly comparable reservoir influence since 2003. We hypothesized that watershed-scale differences in agricultural non-point-source pollution account for these contrasting responses. To test this hypothesis, we integrated the Soil and Water Assessment Tool (SWAT) with partial least squares structural equation modeling (PLS-SEM) to link land use/land cover, hydrology, and nutrient transport to river nutrient enrichment. The agriculture-dominated Pengxi watershed showed higher nutrient export than the more forested Modao watershed. PLS-SEM further showed agricultural land cover had the strongest positive pathway association in the Pengxi watershed (β = 0.731, p < 0.001), whereas forest land cover showed a strong negative pathway association in the Modao watershed (β = -0.812, p < 0.001), consistent with a hydrological buffering role.. These results indicate that watershed land use/land cover composition, rather than reservoir hydrodynamics alone, helps explain differential trophic responses among reservoir tributaries, and they support targeted land use management to mitigate eutrophication in reservoir-affected river systems.