Sediment dredging effects on nitrogen and phosphorus release in a stratified reservoir: Implications for internal...

Sediment dredging efficacy for controlling internal nitrogen (N) and phosphorus (P) loadings in deep-water ecosystems remains elusive. To provide a scientific basis for potential future dredging plans, this study employed an in situ dredging simulation experiment in a seasonally stratified reservoir. Based on intact sediment core incubation, high-resolution Peeper (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to quantify the impact of dredging on internal N and P loading. The results indicated that TN and TP contents decreased by 48.4 % and 50.1 %, respectively, with declines observed across all N and P forms after dredging. In addition, the microbial community structure did not cause any long-term damage. NH₄⁺-N fluxes were enhanced owing to the exposure of the ammonia-rich layer in the early stage (86.38 mg m^-2 d^-1 and 193.88 mg m^-2 d^-1 in non-dredging (ND) and dredging (D) treatments, respectively). Progressive organic matter depletion coupled with sustained pore water diffusion following dredging collectively contributed to the transition into an effective control period for NH₄⁺-N release after 110 days. SRP fluxes ranged from -1.82 ± 0.45 to 1.32 ± 0.48 mg m^-2 d^-1 and -0.62 ± 0.45 to 2.76 ± 0.62 mg m^-2 d^-1 over the experimental period for the D and ND treatments, respectively, with a significant reduction (p < 0.05) after dredging. These results were mainly attributed to the reduction in Fe-P and Org-P after dredging, as well as the reduction in oxygen consumption in the nascent sediment-water interface and organic matter in the dredged sediments, collectively inhibiting Fe-P mobilization and Org-P mineralization. These findings highlight the feasibility of using sediment dredging to control internal loading in deep-water ecosystems, while also considering the potential environmental risks associated with elevated NH₄⁺-N release in the early stages after dredging.