Significant amounts of nitrogen and phosphorus in sediments will be released into the overlying water during the flood season in the water level fluctuation zone (WLFZ) of reservoirs that undergo periodic drying and flooding. This will result in water quality deterioration of the reservoir. In order to clarify the distribution characteristics and release behavior of nitrogen (N) and phosphorus (P) from sediments in the WLFZ of a reservoir, this study analyzed the sediment distribution characteristics and potential exchange flux sediment–water interface(SWI) through field investigations and sediment core incubation experiments. And the main factors affecting the release of N and P through the incubation experiments in sediments of the WLFZ in the reservoir were determined. Our findings indicated that the sediment in the WLFZ serves as the primary source of NH₄⁺-N and acts as a sink for NO₂-N in the overlying water of sediment. The concentration of NH₄⁺-N in the interstitial water of sediments is the key factor that affects the water quality of Biliuhe Reservoir. Total nitrogen content of surface sediments in the WLFZ of Biliuhe Reservoir ranges from 1052.52 ± 49.39 to 3520.54 ± 30.31 mg/kg. High concentrations of N pollution are the primary increased risk of eutrophication in Biliuhe Reservoir during summer. The sediment N and P release flux of BLH1 located in the main stream is 1.67 ± 1.06 and 12.32 ± 2.42 mg·(m²·d)⁻¹, respectively, which is smaller than that of BLH2 (3.27 ± 2.15 and 15.19 ± 2.36 mg·(m²·d)⁻¹, respectively), BLH3 (4.24 ± 1.74 and 17.02 ± 2.47 mg·(m²·d)⁻¹, respectively) and BLH4 (7.78 ± 2.03 and 20.56 ± 2.38 mg·(m²·d)⁻¹, respectively) located in the tributary. It indicates that the water conveyance project located in BLH1 has an impact on nutrient scouring of sediments in the WLFZ at this site. The main water environment factor affecting the release of N and P in the sediment of the WLFZ is dissolved oxygen (DO). And the Pearson correlation coefficients between TN and TP with DO were −0.838 and −0.777, respectively (p < 0.05). At the same time, the diffusion of nutrients in the sediments can be effectively inhibited by maintaining a certain DO concentration in the overlying water.

Climate-sensitive ice-covered reservoirs are critical components of methane (CH₄) release. To reveal the spatial characteristics of CH₄ concentrations, diffusive fluxes and bubble fluxes during the ice-covered and ice-free periods in northern reservoirs, and in order to clarify the critical influences on their variations. We selected Dongfeng Reservoir, a large reservoir in Northeast China, and conducted six field investigations of CH₄ concentrations and emissions in deep and shallow waters during the ice-covered (January 2022 and January 2023) and ice-free (July 2022, October 2022, March 2023, and September 2023) periods. The results showed that spatially, surface CH₄ concentration and diffusive flux were significantly higher in shallow water than those in deep water. CH₄ bubble flux had the largest range of variation in shallow water, while there was no obvious spatial difference in the proportion of CH₄ in bubbles. Temporally, surface CH₄ concentration, diffusive flux, bubble flux, and the proportion of CH₄ in bubbles were generally high in summer and low in autumn. The surface CH₄ concentration had the largest range of variation in winter, and the CH₄ concentration under the ice was significantly higher in shallow water than those in deep water. Water depth determines the release of CH₄ bubbles from sediments and is the basis for determining deep and shallow water based on bubbles. Ice cover leads to significant differences in CH₄ production and transport compared with ice-free periods by indirectly changing the water environment and directly altering the CH₄ release. CH₄ accumulated under the ice and in the ice will greatly increase the CH₄ release potential during the spring ice-melt period. Overall, this study improves the understanding of CH₄ emissions from reservoirs characterized by ice-covered periods and provides theoretical basis for comprehensive estimation of CH₄ emissions from reservoirs.