A comparative analysis of phosphate occurrence and its genesis during different dispatching periods of representative level I tributaries of the Three Gorges Reservoir was conducted. For this, water quality, hydrodynamic force, and environmental data were examined for backwater areas of the Xiangxi River and Shennong River during the low-water-level period (June), storage period (September) and high-water-level period (December) in 2018. The results suggest that the mass concentration of total phosphorus (TP) in the water body of the coves of Xiangxi River and Shennong River range from 0.049 mg·L-1 to 0.168 mg·L-1 and 0.059 mg·L-1 to 0.152 mg·L-1, respectively, surpassing the 0.02 mg·L-1 threshold for algal blooms. The mass concentration of TP, DP (orthophosphate), and PP (particulate phosphorus) in the coves of the tributaries were ranked as follows:storage period > high-water-level period > low-water-level period for TP; storage period > low-water-level period > high-water-level period for DP; low-water-level period > high-water-level period > storage period for PP. Based on Pearson's correlation coefficients, TP and DP were significantly correlated as well as temperature and pH, which are the key factors influencing the phosphorus in soils and sediments in the water-level-fluctuation zone. During the low-water-level and high-water-level periods of the Three Gorges Reservoir, the TP in the coves of the tributaries mainly exists as PP, accounting for more than 75% and 60%, respectively. The flow in the coves of the tributaries slows during storage periods, meaning that settlement of PP is enhanced, changing the dominance of TP to dissolved total phosphorus (DTP).
Keywords: Shennong River; Three Gorges Reservoir; Xiangxi River; high-water-level period; low-water-level period; orthophosphate; particulate phosphorus; storage period; total phosphorus.