River-Lake States in the Tributary of the Three Gorges Reservoir Area and Their Effects on the Phosphorus Content of Different Forms in the Sediment

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The unique operation mode of the Three Gorges Dam has determined that the tributaries of the Three Gorges Reservoir (TGR) are significantly different from natural rivers in terms of hydrodynamics. The aim of this study was to explore the dynamic changes of internal phosphorus in tributary sediments under special water transfer mechanisms in the reservoir area. In 2016, we conducted samplings eight times (January, March-August, and October, once a month) in the Gaoyang Lake, which is located in the middle section of the Pengxi River, the largest tributary in the northern bank of the TGR. Samples of the adjacent overlying (AOL) water and sediment were collected, and a series of analyses were carried out on the flow velocity, water depth, and alkaline phosphatase activity (APA) of sediments. The results indicated that the waterbody types of the Gaoyang Lake can be divided into two hydrological states:deep lakes before May and June and typical rivers from May and June until the Three Gorges Dam re-flooding in autumn. The concentration of dissolved phosphorus in the AOL water during the lake state was 42.48% higher than that in the river state, which was beneficial for the weakly bounded phosphorus (NH4Cl-P) in the sediment surface to be in an adsorption-releasing equilibrium state. At the beginning of the spring algal bloom, with the increase in water temperature and APA in the sediments, the content of potential mobile phosphorus (PMP) in the sediments increased; however, the stable-form phosphorus content decreased, showing a tendency of transformation between these two forms of phosphorus, which may be one of the available phosphorus sources for algal blooms. The stable-form phosphorus content in the sediments of the Gaoyang Lake accounted for approximately 81.79% of the total phosphorus content, its coefficient of variation was relatively small (2.90%-4.21%), and there was a significant trend of accumulation over time (P<0.05). The study revealed the transformation of different phosphorus forms in sediments and their relationship with the state shift of the waterbodies, and provided a reference to understand the internal phosphorus cycle in other tributaries of the TGR area or other reservoir tributaries under similar situations.