Larger phosphorus flux triggered by smaller tributary watersheds in a river reservoir system after dam construction

Water level fluctuation and modification in human activities induced by dam construction have significantly affected the production and input of nutrients in the tributary watersheds. In this study, numerical simulation and time series analysis methods are used to explore the input of phosphorus (P) in different types of tributary watersheds around the reservoirs in river reservoir systems and the impact of dam construction on reservoir water quality. Watersheds within a reservoir were divided into larger and smaller tributary watersheds according to whether their area was greater than or less than 1000 km2. The results showed that smaller tributary watersheds, which account for 29% of the area of the Three Gorges Reservoir Region (TGRR), transported approximately 36%~45% of P and 43.7%~47.8% of sediment deposited into the mainstream from 2003 to 2017. P from the smaller tributary watersheds of the reservoir had a stronger influence on the aquatic environment of the river-reservoir system than that from the larger tributary watersheds. In addition, the annual mean contributions of the P inputs from the “three rivers” (the Yangtze River, the Jialingjiang River and the Wujiang River) and tributary watersheds within the reservoir were 68.76% and 31.18% from 2003 to 2017, respectively. The dam altered the composition of the form of P in the river-reservoir system, and the waterbody close to the dam was identified as the main area containing dissolved P (DP) enrichment. Accordingly, the risk of eutrophication remains high in head area of the dam due to the relatively high and stable P concentration. This study provides critical information for the management of nutrients in a river-reservoir system and the prevention of eutrophication of water bodies.