Water level rising in flood season regulates the temperature-nutrient-phytoplankton aspects: Evidence from a Three Gorges Reservoir tributary.

Understanding the effects of a hydrological regime on the eco-environment of a tributary in a river-type reservoir is essential because reservoir regulations substantially change the freshwater ecosystem in the tributary. The construction of cascade reservoirs upstream of the Yangtze River has significantly altered the water regime and impacted reservoir ecosystems. However, the effects of water level rising (WLR) on the temperature-nutrient-phytoplankton aspects lack research. A three-dimensional model, covering the Three Gorges Reservoir (TGR) and one of its tributaries, the Xiangxi River (XXR), was built based on the Environmental Fluid Dynamics Code (EFDC). The model was calibrated and validated using measured data during 2018 and 2019 and adopted to determine the associations between reservoir operation and the tributary eco-environment. The spatiotemporal distributions of hydrodynamics, temperature, nutrients, and phytoplankton biomass were numerically analyzed under various WLR scenarios during the flood season. The results indicated that WLR intensified mid- and bottom-layer reversed-density currents, decreased water temperature, and, hence phytoplankton biomass, which increased total nitrogen (TN) and total phosphate (TP). 5-m WLRs of 15-day duration reduced Chl-a concentrations by up to 6.18, 2.53, and 0.83 μg/L, in June, July, and August, respectively. Higher magnitudes and shorter durations enhanced the reduction effect of WLR on Chl-a. A 1-m WLR over 15 days resulted in a 0.72 μg/L reduction. These findings suggest WLR as a viable strategy for phytoplankton biomass regulation and algal bloom control in river-type reservoirs.