Effects of Potamogeton crispus on Hydrodynamics and the Distribution of Nitrogen and Phosphorus Nutrients in Meiliang Bay, Lake Taihu

Restoration of aquatic vegetation is a crucial approach for improving water quality and achieving ecological restoration in eutrophic shallow lakes. However, existing studies have largely focused on the impacts of aquatic vegetation on local hydrodynamic conditions and water quality, while the mechanisms by which vegetation alters the spatial distribution of nutrients within large-scale shallow lake circulations remain poorly understood. This study focuses on the dominant submerged macrophyte, Potamogeton crispus, in the Meiliang Bay of Lake Taihu. Considering only its flow resistance effect, a coupled hydrodynamic-water quality model was constructed. By simulating scenarios with and without P. crispus, we analyzed its influence on the distribution of nitrogen and phosphorus nutrients within the circulation. The results indicate that the presence of P. crispus reduced flow velocity by 10%–20% in the vegetated areas but did not alter the position or direction of the counterclockwise circulation in the bay. The impact of P. crispus on nutrient distribution was diametrically opposite on the eastern and western sides of the counterclockwise circulation. The eastern side of the northern Meiliang Bay circulation is characterized by high flow velocity and high nutrient concentrations. Under the combined action of pump station drainage and circulation, water carries a large load of nutrients through this area. P. crispus reduced flow velocity and weakened the water transport capacity, decreasing the average net input flux of nitrogen and phosphorus by 8.67% and 9.27%, respectively. Consequently, the Total Nitrogen (TN) and Total Phosphorus (TP) concentrations in this area decreased by a maximum of 0.0878 mg/L (a 3.2% reduction compared to the no-vegetation scenario) and 0.0274 mg/L (an 18.5% reduction), respectively. Conversely, the western side is a low-flow, low-nutrient zone located at the center and end of the circulation. Here, P. crispus further reduced flow velocity and prolonged water retention time, decreasing the average net output flux of nitrogen and phosphorus by 25.09% and 30.77%, respectively. This led to a maximum increase in TN and TP concentrations of 0.2513 mg/L (a 9.8% rise) and 0.0133 mg/L (a 22.5% rise), respectively. This study reveals the mechanisms by which submerged vegetation alters the spatial distribution of lake nutrients by affecting nutrient transport processes within the circulation, providing a scientific basis for the restoration and optimized management of aquatic vegetation in eutrophic shallow lakes.