Effects of Water-Sediment Regulation on spatial-temporal distribution of nutrients in the lower Yellow River

The operation of the reservoirs in the Water-Sediment Regulation Scheme (WSRS) controls the inflow of water and sediment to the lower Yellow River (LYR). This scheme lead to complex effects on nutrient distribution in space and time, due to the interactions among the main river channel and its tributaries. Twelve observation points were set up from Xiaolangdi (XLD) to the Yellow River Estuary (YRE) during June to August 2023. These sites were tasked with monitoring levels of nitrogen (N) and phosphorus (P) across three stages: These include the water regulation stage, the sediment regulation stage and the after-WSRS stage. The findings showed that this segment of the river is largely nitrogen enriched and phosphorus depleted, with nitrate nitrogen (NO₃⁻-N) and dissolved inorganic phosphorus (DIP) being the dominate forms. The Yiluo, Qin and Dawen rivers, as important tributaries, play a major role in modulating nutrient levels in the LYR as both sources and sinks. The nitrogen and phosphorus fluxes during the WSRS stage accounted for 15.21% and 17.33%, respectively, of the annual totals. We observed a robust linear relationship between the fluxes and relative concentrations of these nutrients. The Class II standard was not consistently met in this segment, indicating that the short term ecological effects of the WSRS required attention. At the YRE, the ratio of dissolved inorganic nitrogen to phosphorus (DIN/DIP) exceeded the Redfield ratio, and these potentially threaten the marine ecosystem. This investigation helps to better understand the ecological consequences of the WSRS on the LYR.