Impact of cascade reservoirs on nutrients transported downstream and regulation method based on hydraulic retention time.

Cascade reservoirs construction has modified the nutrients dynamics and biogeochemical cycles, consequently affecting the composition and productivity of river ecosystems. The Jinsha River, as the predominant contributor to runoff, suspended sediment (SS), and nutrients production within the Yangtze River, is a typical cascade reservoir region with unclear transport patterns and retention mechanisms of nutrients (nitrogen and phosphorus). Furthermore, how to regulate nutrients delivery in the cascade reservoirs region is also an urgent issue for basin water environment study. Therefore, we monitored monthly variations in nitrogen and phosphorus concentrations from November 2021 to October 2022 in the cascade reservoirs of the Jinsha River. The results indicated that the concentrations and fluxes of total phosphorus (TP) and particulate phosphorus (PP) decreased along the cascade of reservoirs, primarily due to PP deposited with SS, while opposing trends for total nitrogen (TN) and dissolved total nitrogen (DTN), which might be the consequences of human inputs and the increase of dissolved inorganic nitrogen discharged from the bottom of the reservoirs. Moreover, the positive average annual retention ratios for TP and PP were 10% and 16%, respectively, in contrast to the negative averages of -8 % for TN and -11% for particulate nitrogen (PN). The variability in runoff-sediment and hydraulic retention time (HRT) of cascade reservoirs played crucial roles in the retention of TP and PP. A regulatory threshold of HRT = 5.3 days in the flood season was obtained for controlling the balance of TP based on the stronger relationship between HRT and TP retention ratio. Consequently, the HRT of these reservoirs could be managed to control nutrients delivery, which was of particular significance for basin government institutions. This study enhances our comprehension of how cascade reservoirs influence the distribution and transport patterns of nutrients, offering a fresh perspective on nutrients delivery regulation.