Study on nutrient variation in mainstream and reservoir system of the middle and lower Yellow River

To study the concentration and composition of nutrients in surface water of the middle and lower reaches of the Yellow River as well as the release or retention effects of Sanmenxia and Xiaolangdi reservoir on nutrient transport, sampling was carried out in the middle and lower reaches of the Yellow River and Xiaolangdi reservoir in June 2017. The results show that in the mainstream of the Yellow River, the average NO₃⁻, dissolved inorganic nitrogen (DIN) and total dissolved nitrogen in the Xiaolangdi reservoir were slightly lower than other stations, and the dissolved phosphorus and dissolved silicon (DSi) were only 41%~60% of other stations, but dissolved organic nitrogen (DON) was significantly increased in the reservoir and the average concentration was 1.2 times of other stations. The concentration of DIN decreased in the tail of the Xiaolangdi reservoir (away from the dam) while the concentration of DON increased, which may be caused by the growth and reproduction of phytoplankton. The concentrations of dissolved inorganic nutrient increased while the concentrations of dissolved organic nutrient decreased in the head of the reservoir (near the dam), which may be caused by the degradation of organic matter, as well as the nitrification of nitrogen (for NO₃⁻) and the dissolution of biogenic Si particles (for DSi). The proportion of dissolved organic nutrients in Xiaolangdi reservoir was higher, and the ratio of nitrogen, phosphorus and silicon was more unbalanced than that of the mainstream, but the lack of water and sediment regulation event in 2017 prevented the large-scale discharge of the significantly unbalanced water body of Xiaolangdi reservoir in the short term, reducing the potential possibility of eutrophication in the downstream, estuary and Bohai Sea. In addition, Sanmenxia and Xiaolangdi reservoir played different roles in nutrient transportation in this investigation, Sanmenxia reservoir had an interception effect on outflow flux, while Xiaolangdi reservoir had a release effect on the transportation of downstream nutrients, and it had increased by 127%~438% compared with the outflow flux of Sanmenxia.