Decomposing riverine nitrogen and phosphorus fluxes into different frequency classes along the Yangtze River, China

Enhanced riverine nitrogen (N) and phosphorus (P) exports from anthropogenic activities have substantially increased primary productivity in downstream waters and induced harmful ecosystem effects. The components of riverine nutrient fluxes determine environmental responses that remain largely unknown. We identified different components of riverine N and P exports based on a load-hydrograph analysis of multiple sections of the Yangtze River in China based on long-term daily nutrient fluxes. Our results indicate that the increasing trend of riverine N and P fluxes from upstream to downstream can be reversed by the retention effect of dams and lakes, which is more significant for total phosphorus and its high-flux component than total nitrogen. The greatest nutrient retention along the river was mainly attributed to the Three Gorges Dam, which has a significant retention effect on both N and P fluxes, particularly on the high flux and TP. While high nutrient fluxes dominate upstream, middle and low fluxes dominate downstream. Significant but uncommon trends were observed for all nutrient flux components along the river. While both, medium and low flux percentages increase significantly, those of high flux decrease. The net change of N and P fluxes along the Yangtze River do not coincide in space, indicating heterogeneity between the river's source and sink of N and P. Knowledge of the inconsistent alteration of riverine nutrient flux and its components should facilitate efforts to make better measures to mitigate nutrient-related problems in the Yangtze River.