Dynamic Simulation and Causal Mechanisms of Eutrophication in Irregular Shallow Urban Lakes Based on Numerical Models

Urban irregular shallow lakes are characterized by sinuous shorelines and substantial variations in water depth, which promote the accumulation of pollutants and nutrients. The interaction between nonpoint source pollution and internal pollution leads to rapid and frequent occurrences of eutrophication, complicating the prevention and remediation endeavors. To explore the mechanisms and causes of eutrophication under such circumstances, a two-dimensional hydrodynamic-water quality coupled model of Meijiang Lake was established using the MIKE 21 numerical model. A series of extensive field measurements conducted from April to September 2024 were utilized for model parameter calibration and validation. By simulating the spatiotemporal distribution of lake flow fields and key water quality parameters, such as total nitrogen, total phosphorus, and chlorophyll a, under both inflow and stagnant conditions, and by introducing the lake shape index (LSI) to quantify lake morphology, this study systematically analyzed the influence of lake shape on hydrodynamic conditions and the risk of eutrophication. The results suggest that the model demonstrates high reliability (Nash coefficient close to 0.9). Simulations reveal notable shape-dependent hydrodynamic effects. Lake A, with a regular shape and an LSI of 1.9, maintains optimal water quality owing to limited external inputs and strong water exchange capacity. Lake C, also with a regular shape and an LSI of 1.7, attains moderate water quality despite relatively high external inputs, benefiting from its robust exchange capacity. Lake B, featuring a tortuous morphology and an LSI of 4.65, exhibits low flow velocity and poor water exchange capacity. Coupled with high external inputs, this results in the formation of local stagnant areas with severe nutrient accumulation, rendering it a high-risk area for algal bloom outbreaks. Correlation analysis and quantitative results further demonstrate that chlorophyll a concentration is positively correlated with LSI, water temperature, and total phosphorus and negatively correlated with flow velocity and dissolved oxygen. With each one-unit increase in LSI, the lake's average flow velocity decreases by 35%, while the average eutrophication level rises by approximately 4.1% (range: 1%-7%). This confirms that lake morphology is one of the key factors regulating differences in eutrophication, providing a scientific basis for differentiated management and ecological restoration of urban landscape lakes.