Spatial and historic changes of nutrients across the changjiang river and associated major tributaries

Nutrients concentrations and fluxes in large rivers are substantially enhanced by human activities and global changes. Spatial and historical changes of nutrients in large rivers, however, remain poorly understood. Here, we investigated and summarized dissolved NH₄⁺, NO₃^-, NO₂^- (DIN), phosphorus (DIP), and silicon (DSi) concentrations and fluxes in the Changjiang River and associated eight major tributaries. DIN concentrations were significantly higher in midstream and downstream than upstream, and DIP and DSi concentrations were significantly higher in downstream than upstream and midstream. DIN/DSi, DIN/DIP and DSi/DIP ratios were observed to increase from headwaters to the Three Gorges Dam (TGD) then significantly decreased to estuary of the Changjiang River. The nutrient ratios also increased after construction of the TGD, further suggesting water quality degradation under the declining water discharge. DIN variations highly coupled with DIP, indicating similar sources along the Changjiang River and tributaries. DIN and DIP concentrations were significantly lower in Yalongjiang and Minjiang Rivers than other tributaries, and DSi concentrations were comparable across the eight tributaries. Annual DIN and DIP fluxes increased by one order of magnitude during 1980-2020, ranging from 30.5 to 177 Gmol yr^-1 and from 0.18 to 2.29 Gmol yr^-1, respectively, mainly due to nitrogen and phosphorus fertilizers from agricultural activities and domestic sewage discharge from urban expansion along the Changjiang River. Annual DSi fluxes decreased substantially from 137 to 78.9 Gmol yr^-1, which were correlated with the decline of suspended sediments caused by large-scale afforestation project and the TGD. These results suggested that DIN and DIP loadings have increased substantially and DSi loadings has decreased over the past four decades. Human activities, dam construction as well as national eco-friendly policies have collectively exerted a profound influence on nutrients ratios and transportations in the Changjiang River.