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 8.97 similar to 74.84 mgd(-1) m(-2), higher in the middle of the lake, followed by the northern and southern regions successively. The P diffusive flux is -0.007 similar to 0.050 mgd(-1) m(-2), 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.