Impact of a water-sediment regulation scheme on nutrient variations at the Lijin station of the Yellow River

Abstract

The water-sediment regulation scheme (WSRS) imposed on dams throughout the Yellow River not only alleviates siltation in the downstream section but also alters the nutrient characteristics, which indirectly affects the enrichment of nutrients in the estuary. Nevertheless, the long-term changes in the nutrient contents and their causes in the lower Yellow River (LYR) remain unclear, and the nutrients characteristics during the years with and without WSRS have yet to be compared. Therefore, the purpose of this study was to explore the variations in the nutrient contents and limitations at the Lijin station on the LYR over the past decade, especially during the annual WSRS period, and to compare the water quality characteristics at Lijin between the years with and without WSRS. The results reveal that WSRS significantly changed the seasonal nutrient concentrations (nitrogen, phosphorus and silicon) at the Lijin station. The fluxes of these nutrients during WSRS (excluding 2016 and 2017) accounted for 11.64–40.63% of the total annual fluxes. The N concentration in the LYR was higher than that in some global rivers, while the concentrations of dissolved inorganic phosphorus (DIP) and dissolved silica (DSi) were lower than the average levels in other rivers. In addition, higher values of dissolved inorganic nitrogen (DIN), DSi and the Redfield ratio indicated that the growth of phytoplankton at the Lijin station was strongly restricted by P. However, during the 2 years without WSRS (2016 and 2017), the proportions of the nutrient fluxes in June were less than 66% of those in the WSRS period in other years. Additionally, there was a potential Si limitation in June in these 2 years. Furthermore, due to the occurrence of floods upstream of the Yellow River and the low-level operation of the Xiaolangdi Reservoir, the fluxes of nutrients during WSRS in 2018 were approximately 0.90–4.20 times those during the same period in 2009–2015 and 6.30–35.76 times those in June 2016 and June 2017. This study shows that WSRS effectively changes the nutrient balance in the LYR and provides a reference for the multi-objective collaborative optimization of WSRS to improve siltation and control flood in the LYR.

Atypical Seasonality of the Silicon Cycle in the Yellow River Estuary and Bohai Sea Revealed by Stable Silicon Isotopes

“…Given the large DSi consumption (>70%) observed in winter and spring relative to the mixing line, the Rayleigh model is adopted to calculate the initial δ 30 Si DSi values prior to diatom production via Equations 8 and 10. In summer and autumn, the Rayleigh model gives similar results to that of the steady state model due to the (Li et al, 2022), which directly affects the calculation of the f values. The range of the DSi concentrations of the Yellow River leads to variation of the resulting initial δ 30 Si values by less than 0.2‰ via both models, which is negligible given the analytical errors of Si isotope analysis.…”

“…This means that the δ 30 Si DSi values above the mixing line in the estuary right after the WSRS in summer (Figure 4b) are likely originated from land instead of in situ primary production in seawater. This is also supported by synchronous low DSi concentrations of the Yellow River water (Li et al, 2022) and low Chl a concentrations of the Bohai seawater (Cui et al, 2024). In addition, due to the long residence time of freshwater, diatom production in summer could be suppressed by the pollution of atrazine that is heavily used in spring, but this effect is only detected within limited areas (Yang et al, 2021).…”

“…Delivery of the suspended river sediments derived from land also greatly increases water turbidity of the Yellow River mouth, which in turn suppresses primary production. The excessive DSi supply during the WSRS evidenced by a high DSi/DIP ratio of >100 (Li et al, 2022) directly turns the nutrient limitation from DSi in spring to DIP in summer (Figure 5), which suggests a clear nutrient regime shift as a result of the WSRS. Furthermore, water in the reservoirs behind dams often has high BSi concentrations and δ 30 Si DSi values given ongoing biological activity.…”

“…10.1029/2023GB007894 75 μmol/L in summer after 2013, except in 2016 and 2017 when the WSRS was paused for two years (Li et al, 2022). By taking seasonal variations of DSi concentrations into account, we used the DSi concentration of 96 ± 20 μmol L 1 to cover the major variation range of the Yellow River endmember.…”

Regime of fluvial phosphorus constituted by sediment

“…In recent decades, sediment has been intensively disturbed by human activities, such as river damming and inter-basin water diversion projects (Vörösmarty et al, 2003;Syvitski et al, 2005;Best, 2019;Grill et al, 2019), which inevitably affects P transport and transformation processes in rivers (Zhou et al, 2013;Maavara et al, 2015;Zhou et al, 2015;Li et al, 2022). For instance, the substantial reduction in total P (TP) trapped by dams is widely observed in rivers, reaching 77% in the Yangtze River (Zhou et al, 2013), 40% in the Madeira River (Almeida et al, 2015), and 60% in the Zambezi River (Kunz et al, 2011), mainly due to the reduced sediment loads.…”