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

Abstract

 Cascade reservoirs construction has modified the nutrients dynamics and biogeochemical cycles, consequently affecting the composition and productivity of river ecosystems. Cascade reservoirs in different rivers typically exhibit distinct variabilities in the retention characteristics of different nutrients. 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). Therefore, we monitored monthly variations in nitrogen and phosphorus concentrations from November 2021 to October 2022. The results demonstrated that the concentrations and fluxes of total phosphorus (TP) and particulate phosphorus (PP) significantly decreased as they moved downstream along the cascade of reservoirs, primarily due to PP deposited with SS, while total nitrogen (TN) and dissolved total nitrogen exhibited opposing trends. Moreover, the positive average annual retention rates for TP and PP were 9.64% and 15.64%, respectively, in contrast to the negative averages of -8.38% for TN and -10.51% for particulate nitrogen. A higher proportion of TP and PP was retained by the reservoirs in the flood season compared to the non-flood season. Additionally, the variability in runoff-sediment and hydraulic retention time (HRT) of cascade reservoirs played crucial roles in the retention of TP and PP. A stronger relationship between HRT and TP retention rate during the flood season suggested that the cascade reservoirs could effectively transport or intercept TP downstream when HRT was either less than or greater than 5.3 days. Consequently, the HRT of these reservoirs could be managed to control nutrients delivery, which was of particular significance for watershed 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.