Evaluation of sediment phosphorus dynamics in cascade reservoir systems: A case study of Weiyuan River, China

Reservoirs tend to accumulate phosphorus (P) originating from agriculture, industry, and other upstream sources in sediment, with this stored P later released. However, the spatiotemporal dynamics of sediment P release in reservoirs remains unclear. This study investigated the spatiotemporal dynamics in P of the sediment and water of three cascade reservoirs in the Weiyuan River (Tuojiang tributary). The results showed elevated P in sediment [total P (TP): 1208.93 mg kg^-1] and water (TP: 0.23 mg L^-1) during the low-water season (LWS), which could be attributed to notably higher organic matter content (9.65%), finer particle size (20.95 μm), and extended hydraulic retention time (HRT: 13.13 days) downstream of the cascade reservoirs. Further study employing static in-situ diffusive gradient in thin films (DGT) and dynamic ex-situ adsorption kinetic experiments confirmed that the downstream release of P from sediments [diffusion flux (F_d): 1.67 mg m^-2 d^-1, equilibrium P concentrations (EPC₀): 0.22 ± 0.10 mg L^-1] greatly exceeded those upstream (-0.66 ± 0.17 mg m^-2 d^-1, 0.07 ± 0.001 mg L^-1), Fe (II) was a critical factor in regulating sedimentary P release. The combined effects of high P in overlying water and sediment significantly stimulated downstream phytoplankton growth, particularly among cyanobacteria (26.48%) and green algae (8.33%). Further regulatory steps are needed to regulate LWS algal blooms downstream of cascade reservoirs.