Spatial-temporal distribution of sediment phosphorus with sediment transport in the Three Gorges Reservoir.

Suspended sediment is an important phosphorus (P) adsorption medium in river catchments. Early adsorption isotherm models often ignored sediment heterogeneity, resulting in incorrect sediment P estimates in field environments. In the Three Gorges Reservoir (TGR), China, P load assessment is essential to eutrophication risk management, but the sediment P evolution in the TGR is unclear. Herein, the P-adsorption capacity of suspended sediment was estimated with an improved Langmuir model via sediment parameter consideration, and the long-term distribution and variations in simulated sediment P with sediment transport were assessed from 2003- 2016. The results showed that the improved Langmuir model attained a good fit with experimental and field data. The sediment load entering the TGR significantly decreased, especially the median size (D50) fraction smaller than 0.008 mm, resulting in long-term discharged sediment load decline and annual mean D50 increase. Meanwhile, the annual sediment P load in the TGR decreased from 7.46- 22.38 kg/s in 2003 to 1.74- 4.73 kg/s in 2016. The increasing sediment particle size reduced the sediment P load and was sensitive to the low sediment P load in the regular impoundment stage (September 2008- 2016). The flood season (June-September) transported 69.2- 98.6% of the annual sediment P. Around 62.3% of total sediment P load was retained in the TGR from 2003- 2016. The results revealed that the retention role of the Three Gorges Dam (TGD) facilitated the long-term reduction in fine sediment and sediment P in the TGR downstream. This study highlights the importance of the particle size in P-adsorption capacity estimation with suspended sediment transport.