Nutrient dynamics in the Yellow River -a case study of different reservoir regulation operations

Global dam construction and reservoir artificial regulations change the biogeochemical processes of nutrients along river networks. Herein, nutrients and hydrologic parameters were analyzed to ascertain their spatiotemporal heterogeneity in the Yellow River and assess the retention effect of nutrients in the Xiaolangdi Reservoir (XLDR). Nutrients increased upstream of reservoirs (the Sanmenxia Reservoir and XLDR) due to the inflow of tributaries (Wei River and Fen River). The XLDR maintained a balanced effect on most nutrient levels from upstream of reservoirs to downstream by the disparities in biogeochemical processes during various periods, including biotic assimilation consumption in the upper layer and regeneration and release in the lower layer during the Drainage period; limited water residence time of the XLDR and constrained biotic assimilation processes in the Flood control period; vertical water mixing in the Impoundment period. The disparities of most nutrient levels upstream of reservoirs played a crucial role in nutrient characteristics in the Yellow River during various periods. The higher DSi levels derived from enhanced physical erosion and chemical weathering of rock/ soil responded to the higher water discharge and suspended particulate matter levels upstream and in the Yellow River during the Flood control and Impoundment periods; however, the lower DIN (dissolved inorganic nitrogen) and TDP (total dissolved phosphorus) resulted from the point pollution source and amplified water dilution effect. We demonstrated that the XLDR acted as a nutrient source for DIN, DIP and DSi based on the mass balance model in a hydrological year (June 2017-May 2018), with retention efficiency (RETf) of-1.4 %,-11% and-7%, respectively. The decreased RETf of DIP primarily stemmed from the pronounced reservoir source effect in the Drainage period. Consequently, the XLDR could relieve the P limitation downstream and estuary by changing the nutrient stoichiometry ratios of outflow.