[Effects of the Three Gorges Reservoir Operation on Vertical Distribution of Chlorophyll a and Environmental Factors in Tributaries]

The hydrodynamics and environmental factors in the Xiangxi River (XXR) and Shennong River (SNR), which are tributaries of the Three Gorges Reservoir (TGR), were monitored from July to August (the low water level period) and in October (the impoundment period) in 2018. The vertical distribution characteristics of chlorophyll a and other indicators of the two tributaries were analyzed during the different operation periods, and the factors that affected the vertical distribution in each period were discussed. The results showed that the vertical distribution of dissolved oxygen, water temperature, pH value, and chlorophyll a of the XXR and SNR during the low water level period was relatively consistent. The indexes 0-10 m (0-5 m for chlorophyll a) from the surface of the XXR and SNR, respectively, showed significant stratification and decreased with increasing water depth; the stability index of thermal stratification (RWCS/H) was 13.71-29.07 m^-1, which was stable. After the water depth reached 10 m (5 m for chlorophyll a), the indexes tended to be stable along the water depth. During the impoundment period, there was no obvious stratification for each index; the stability index of thermal stratification was 0-0.5 m^-1, the stability of the water body was weak, and the vertical variation of each index was relatively stable. The comprehensive trophic state index (TLI) of the XXR and SNR were 55 and 53 during the low water level period, respectively, indicating that they were in a slightly eutrophic state, and 39 and 46 during the impoundment period, respectively, indicating a mesotrophic state. Linear regression analysis showed that chlorophyll a, dissolved oxygen, water temperature, and pH in the two tributaries were significantly correlated in the vertical direction in the low water level period, indicating that dissolved oxygen, water temperature stratification, and pH were important factors affecting the vertical distribution of chlorophyll a. During the impoundment period, a large amount of backflow from the Yangtze River, a large fluctuation in tributary water level, and the decrease in RWCS/H were the important factors that affected the small vertical change in the water body. The enhancement of vertical mixing and the decrease in Z_eu/Z_mix were the key factors affecting the nutritional status of the water.

[Characteristics of Phosphorus Speciation and Genesis in Typical Tributaries of the Three Gorges Reservoir]

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).

[Vertical Distribution Characteristics of Dissolved Oxygen and Chlorophyll a in Typical Tributaries During the Impoundment Period of the Three Gorges Reservoir]

The water quality of the backwater areas in the Xiangxi River and Shennong River, which are typical tributaries of the Three Gorges Reservoir, was monitored in September 2018. The vertical distribution characteristics of dissolved oxygen, chlorophyll a, and other indicators in the two rivers were analyzed and compared, and the environmental factors affecting their vertical distribution were discussed. The results showed that the dissolved oxygen concentration 0-10 m and 0-12 m from the surface of the Xiangxi River and Shennong River, respectively, showed significant stratification and decreased with increasing water depth. The dissolved oxygen saturation of surface water was 139.20% and 107.78%, respectively, reaching a state of supersaturation (S_DO>100%).The dissolved oxygen concentration in the middle and bottom water was more stable without stratification. The vertical distribution characteristics of chlorophyll a were consistent with those of dissolved oxygen in the Xiangxi River and Shennong River, and the chlorophyll a concentration in the surface water showed moderate eutrophication (5μg·L^-1 < Chl-a < 20 μg·L^-1). According to Pearson correlation analysis, the vertical distribution of dissolved oxygen in the Xiangxi River and Shennong River was significantly correlated with that of water temperature and phytoplankton. The stratification of water temperature and the life activities of phytoplankton were the key factors affecting the vertical distribution of dissolved oxygen. Chlorophyll a was positively correlated with water temperature and pH, and negatively correlated with turbidity, indicating that the vertical distribution of phytoplankton was mainly affected by the attenuation of light intensity along the water depth and the stratification of water temperature.