Features and influencing factors of nitrogen and phosphorus diffusive fluxes at the sediment-water interface of Erhai Lake.

Nitrogen and phosphorus diffusion at the sediment-water interface is vital to the water quality of lakes. In this paper, N and P diffusive fluxes at the sediment-water interface in Erhai Lake were studied using the sediment-pore water diffusive flux method. Characteristics of temporal and spatial variation of N and P diffusive fluxes were analyzed. Effects of the physicochemical properties of sediments and overlying water were discussed. Results showed that (1) the total N and P diffusive fluxes at the sediment-water interface of Erhai Lake are relatively low. The diffusive flux of ammonia nitrogen is 8.9774.84 mgd m, higher in the middle of the lake, followed by the northern and southern regions successively. The P diffusive flux is -0.0070.050 mgd m, higher in northern region of the lake, followed by middle and southern regions successively. The annual N diffusive flux has two peaks, and the higher peak is in September. The annual P diffusive flux shows a "V-shaped" variation, reaching the valley in July. N and P diffusive fluxes decrease with an increase of sediment depth. Overall, N and P diffusive fluxes at the sediment-water interface in Erhai Lake show different temporal and spatial variation. (2) Aquatic plants promote N and P diffusion at the sediment-water interface in Erhai Lake. The pH, DO, and SD of the overlying water are important influencing factors for the P diffusive flux. P diffusive flux is inversely proportional to the total phosphorous (TP) concentration of the overlying water. The physicochemical environment of overlying water slightly influences the N diffusive flux. The activity of sediments and the organic content are two main influencing factors of N diffusive flux, while P content and morphology of sediments are the main influencing factors of P diffusive flux. Iron and manganese ions are important elements that influence N and P diffusive fluxes at the sediment-water interface. (3) The P diffusive flux at the sediment-water interface in Erhai Lake is mainly affected by the physical and chemical properties of water, whereas the N diffusive flux is mainly influenced by the mineralization of organic matter in sediments. The P diffusive flux at the sediment-water interface is sensitive to the overlying water quality. Sediment transformation from "source" to "sink" was observed in 1 year. On the contrary, N diffusive flux is less sensitive to lake water quality. Endogenetic pollutant control in Erhai Lake should focus on P control.