Reservoir Dispatching and Basin Dynamics Influence Nutrient Deposition in the Water Level Fluctuation Zone of the Three Gorges Reservoir

Complex nutrient depositional processes in the Three Gorges Reservoir (TGR) are generally explained using hydrological monitoring data or shallow sediment sampling. However, an in-depth understanding of the chronological transformations in the nutrient dynamics of deposited sediment over time is lacking. This study aims to assess the processes and underlying mechanisms of sediment-associated carbon and nutrient deposition in the water level fluctuation zone (WLFZ) of the TGR. Sediment cores were extracted from the WLFZ close to the flood control water level (145.6 m) of the reservoir in 2013 and 2019 and analysed for particle-size distribution (PSD), median particle size (MPS), total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP). Various nutrient ratios (C:N, C:P, N:P and C:N:P) were calculated. The sediment core chronology was determined using ¹³⁷Cs activity. Daily reservoir water levels, monthly suspended sediment loads, reservoir inflow and outflow and upstream reservoir construction data were used to interpret the depositional processes and nutrient distributions. Sediment depths were 345 cm and 440 cm in 2013 and 2019, respectively. Annual and seasonal variations were observed in the sediment deposition, PSD, MPS, ¹³⁷Cs, TOC, TN, TP and nutrient ratios. Mean TOC, TN and TP in 2013 were 18.6, 1.0 and 1.0 (g/kg), respectively, and 16.8, 1.1 and 0.9 (g/kg), respectively, in 2019. The average C:N:P (molar) in 2013 and 2019 were 51:2:1 and 46:3:1, respectively. The controlled flow regime, repeated inundation cycles, input sediment loads and upstream hydrological management primarily govern nutrient transportation and depositional mechanisms. Overall sedimentation decreased over time, and the deposited sediment exhibited a relative increase in fine sediment, TOC, TN and TP owing to coarse sediment retention in the upstream reservoirs. This study highlights complex geochemical alterations in reservoir ecosystems akin to large-scale reservoir operations.