Spatial and temporal changes in nutrients associated with dam regulation of the Yellow River

Abstract

Sediment regulation via manipulating dam operations is an effective solution for dealing with reservoir siltation and downstream channel incision. Large amounts of sediment and thereby nutrients can be discharged within a short period of time. To ascertain the concentration of and transformation between dissolved and particulate nutrients during such periods, we conducted field observations in the Xiaolangdi Reservoir and its downstream reach during the sediment regulation stage of the Water-Sediment Regulation Scheme (WSRS) in 2018. We found that water and sediment characteristics differed significantly pre- and post-WSRS period. Nutrient concentrations varied significantly among the dissolved, suspended, and deposited phases, while no significant differences were observed between the reservoir and downstream reach, except for dissolved total carbon, particulate ammonium nitrogen, and calcium-bound phosphorus. Total nitrogen (TN) and total phosphorus (TP) concentrations of the dissolved phase were significantly positively correlated with suspended sediment concentration (SSC) and electrical conductivity (EC) (p < 0.05). Total organic carbon (TOC) concentration of the suspended phase was significantly negatively correlated with EC and median particle size (D50) (p < 0.05). Nutrient concentration during the deposited phase was less correlated to selected hydrological and physical parameters. Nutrient transformation between the deposited phase and the suspended and dissolved phases were higher than those between suspended and dissolved phases. TP transformation was closely related to hydrological parameters (SSC and D50), while TN transformation was predominantly related to physical parameters (EC and DO). The concentration and transformation of nutrients during the transport process from the reservoir to downstream were likely driven by different factors. In conclusion, reservoir management practices like WSRS could mitigate nutrients retention in reservoir, however, leading to higher nutrients transformation downstream.