Seasonal and decadal variations in nutrients and organic carbon in the lower Huanghe River under drastically changing water-sediment regimes

The Huanghe (Yellow) River is well known for its sediment-laden river waters, and it is also a typical large river deeply imprinted by human activities. Surface water samples were collected monthly from March 2019 to October 2022 at the Luokou station in its lower reaches, to elucidate the seasonal variation patterns of POC (particulate organic carbon) in SPM (suspended particulate matter), nutrients, and DOC (dissolved organic carbon) ready to be transported to the sea. The annual water-sediment regulation scheme (WSRS), generally carried out in June and July since 2002 at the Xiaolangdi reservoir, induced pulsed changes in both water discharge and sediment contents downstream that destroyed the natural sediment-water balance, so that the chemical parameters of sediments (POC (%), POC/PN (particulate nitrogen), δ¹³C in POC, and δ¹⁵N in PN) at the lower reaches did not show clear seasonal variation patterns anymore. At the same time, however, concentrations of NO₃⁻, SiO₃²⁻, NO₂⁻, and DOC still maintained apparent seasonal variations. Concentrations of NO₃⁻, SiO₃²⁻, NH₄⁺, and NO₂⁻ as well as DOC displayed significantly decreasing trends after 2000, which were different from their decadal trends of increasing in the last century. These new trends, owing to the successful implementation of the environmental protection projects, as well as to the prolonged retention times of river waters after the constructions of a large number of reservoirs over the river basin, reflected the profound transformation and evolution of the Huanghe River aquatic ecosystems just occurring recently.