Response of Lake ecosystems to variations in external phosphorus inputs in Poyang Lake

External phosphorus inputs are a primary driver lakes eutrophication. Excessive loading can trigger algal blooms and other ecological disturbances, ultimately leading to ecosystem degradation. Understanding how external phosphorus inputs affect lake ecosystems is therefore essential for maintaining lake health and ecological functions. Most existing studies analyze the response of external phosphorus inputs using a single ecological component to represent the entire lake ecosystem, often overlooking the differential responses of various components. This study established a Poyang Lake ecosystem model (Water Ecosystems Tool, WET) for 2013–2023 to analyze the response patterns of total fish biomass and shallow lake regime shifts to external phosphorus inputs. The results showed that, under the conditions simulated in this study, fish biomass was maintained at relatively high levels when phosphorus (TP) concentrations were within a model-derived indicative range of approximately 0.09–0.17 mg/L. Shallow lake regime shifts displays a nonlinear hysteresis response to changes in nutrient inputs. Within our model framework, the simulated transition points between clear-water and turbid states occurred at TP concentrations of approximately 0.03 and 0.10 mg/L. For shallow lakes in a clear-water state, the model results suggest that TP concentrations of approximately 0.09–0.10 mg/L may be associated with favorable outcomes for both water quality and fish biomass under the conditions simulated in this study. In contrast, for shallow lakes already in a turbid state, no clear optimal ecological range exists, implying that trade-offs between water quality improvement and fishery resources may be necessary. This study enhances the understanding of the trade-offs between fish communities and water quality under different nutrient conditions, and provides scientific guidance for shallow lake ecosystem management and sustainable development.