Flow regulation controls sediment, carbon, and nutrient dynamics across the elevation gradient in the water level fluctuation zone of the Three Gorges Reservoir, China

Purpose Substantial quantities of sedimentary materials are transported from the Yangtze River toward the water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR). Sediment and riparian soil coring across the elevation gradient in the WLFZ of the TGR over a 6-year period are used to interpret deposition patterns, particle size properties, total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), and nutrient ratios using fluvial and suspended sediment data from upstream under flow regime manipulation of the reservoir. Materials and methods Sediment and associated riparian soil cores were extracted in 2013 and 2019 across various elevation gradients in the WLFZ of the TGR. A total of 15 sediments and 24 soil cores were extracted. The sectioned sediment and soil subsamples were analyzed for particle size distribution (PSD) (including sand, silt, and clay fractions); median particle size (MPS) distributions, TOC, TN, TP, and various nutrient ratios were calculated. Upstream suspended sediment and hydrological data were used to interpret sediment deposition, carbon, associated nutrient deposition, and storage dynamics across the elevation gradient. Results and discussion Sediment deposition and relative fining of sediment and riparian soil were irregular, decreased with an increase in elevation, and increased with time. In 2013, the distributions of TOC, TN, TP, and nutrient ratios were irregular, and comparatively stable distributions were observed in 2019, which increased from lower to upper elevation gradient. With a few exceptions, similar trends were also observed in riparian soil core profiles. We conclude that water level fluctuations, water residence time, suspended sediment dynamics, time, and topography played a significant role in determining changes in sediment and riparian soil properties over time. Fine sediment-induced carbon and nutrient lateral translocations and subsequent interactions in sediment and soil layers due to repeated inundation cycles change the sediment and soil properties over time. Conclusions This study interprets sedimentary carbon and nutrient redistribution and translocation processes. The concentrations of carbon and nutrients in sediments and soils change over time due to various fluvial, sedimentary, and geochemical processes, including the hydrological regime, flow regulation manipulation, repeated inundation cycles, water residence time, and suspended sediment dynamics. Fine sediment-associ