Sedimentation substantially buries more carbon than nitrogen or phosphorus in the Three Gorges Reservoir, China

Sedimentation and the accompanying accumulation of nutrients such as total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) pose threats to ecological stability and the sustainable operation of reservoirs. The Three Gorges Reservoir (TGR) in China is a mega river-type reservoir with substantial sedimentation since its impoundment, although the quantity of nutrients buried in the TGR is unknown. In the present study, the monthly suspended sediment loads at the TGR inlet (Cuntan and Beibei hydrologic stations) and outlet (Yichang hydrologic station) were collected during 2003-2023 to estimate the total sedimentation. Time-integrated suspended sediments in the sedimentary channel, discrete riverbed surface sediment throughout the reservoir, and a sedimentary core from a representative channel were sampled and their physicochemical properties determined. Results indicate that the total TGR sedimentation during 2003-2023 was ∼2.06 billion tonnes (Bt), with most (90.78 %) occurring during the wet season. Nutrient concentrations in suspended sediment were significantly higher in the dry season than the wet season. Riverbed surface sediment exhibits a downstream fining trend, without a downstream trend in nutrient content. The sedimentary core has a layered structure due to seasonal sediment accumulation, with nutrient concentrations being significantly lower in the profile than in suspended sediment. Based on the spatiotemporal differentiation in sediment properties and the calculated sedimentation amounts in the TGR, the sedimentary nutrients are estimated to 35.11, 2.14, and 1.95 million tonnes of TOC, TN and TP, respectively. The findings herein highlight that sedimentation burying substantially much more carbon than nitrogen or phosphorus, emphasizing the need for continued monitoring and management strategies to mitigate potential ecological impacts.