The Phosphorus Interception Effect in the Cascade Reservoirs of the Weiyuan River

Abstract

It is widely recognized that the construction of artificial dams has a profound impact on the biogeochemical cycling of phosphorus within river ecosystems. Research on the reservoir's effect on phosphorus retention primarily concentrates on individual dam bodies, while the impact of a multistage sluice dam system on phosphorus retention and its interception function within the same basin remains unclear. This study quantitatively analyzed the chemical forms of phosphorus (TP, Fe (Ⅱ) -P, CDB-P, Ca-P, Loosely-P, and OP) in the sediments of the Weiyuan River step reservoir. Additionally, the retention of phosphorus in the sediments during different water periods[High-water season(HWS),Low-water season(LWS),Normal-water season(NWS)]was investigated. The findings revealed that TP concentration in both overlying water and sediment was highest during LWS (0.17mg/L,1121.72 mg/kg),which could be attributed to the stable hydrodynamic conditions of LWS, small reservoir capacity(1.14million m3), and low flow rate(2.0m3/s). Ca-P dominated in sediment(46%), possibly linked to the high calcium content of the soil in southwest China. Notably, the phosphorus retention in overburden water by the cascade reservoir during LWS(104.7%) far exceeded that of HWS(57.14%). This discrepancy may due to the high P concentration(0.17mg/L) and extended hydraulic retention time(HRT) of 13.13 days during LWS. The concentrations of TP(1004.99 mg/kg) and bioavailable phosphorus (BAP)( 564.08 mg/kg) in the downstream sediments are elevated, designating this area as a phosphorus concentration site. The study results challenges the conventional belief that a singular reservoir is solely responsible for phosphorus retention, it appears that the elevated levels may stem from the collaborative influence of fine particulate matter and organic matter(OM).Consequently, it is imperative to implement additional control measures downstream of LWS to mitigate the risk of reservoir algal blooms.

Keywords: sediment, Overlying water, Hydraulic retention time, particle size, Retained P load, Retention ratio