Longitudinal decoupling between sedimentary phosphorus concentrations and sediment sorting in the Three Gorges Reservoir, China

• Sediments show a downstream fining trend in the reservoir’s permanent backwater reach. • Concentrations of sedimentary phosphorus show no significant downstream variation. • Sediment sorting exerts limited control on distribution of phosphorus concentrations. Fluvial sediments serve as a medium for nutrient transport and burial in river systems. Nevertheless, how sediment sorting within fluvial reservoirs governs longitudinal phosphorus (P) concentration gradients in permanent backwater zones remains unclear. In the present study, sedimentary profiles from the 660-km-long Three Gorges Reservoir (TGR) were measured for particle-size distribution, total carbon (TC), total nitrogen (TN), and six phosphorus species: exchangeable P (Ex-P), aluminium-bound P (Al-P), iron-bound P (Fe-P), bicarbonate-dithionite extractable P (BD-P), calcium-bound P (Ca-P), and organic P (O-P). Sediment particle size analysis revealed a distinct hydrodynamic sorting pattern within the permanent backwater zone, evidenced by a downstream fining trend where median particle size decreased from 16.18 μm (upstream) and 7.45 μm (downstream). Sedimentary profiles further exhibited either downcore coarsening or rhythmic laminations, reflecting distinct depositional histories. The proportions of different P species in TGR sediments varied significantly, with Ca-P dominating (56.73 %), followed by O-P (21.42 %), BD-P (11.47 %), Al-P (5.72 %), and Ex-P (3.01 %), while Fe-P (1.65 %) was the least abundant. No consistent longitudinal trend was observed for the six P species, instead, their concentrations showed a heterogeneous patterning along the TGR’s backwater zones, suggesting a complex control beyond hydrodynamic sorting. Additionally, the concentrations of six P species showed no strong correlation with sediment particle-size composition. These results demonstrate that the longitudinal distribution of sedimentary P concentrations is decoupled from hydrodynamic sediment sorting in the TGR’s permanent backwater zone. The findings herein highlight that the hydrodynamic sorting in the fluvial reservoir caused downstream fining of sediment particles, but it failed to govern the longitudinal distribution of P concentrations through grain-size differentiation. This indicates that sedimentary P distribution in the fluvial reservoir is primarily controlled by allochthonous inputs, suggesting that reservoir management should therefore continue to target external nutrient loads.