Exploring nonlinear responses of hydrodynamics and algal biomass to restoration measures in a eutrophic shallow urban lake.

Shallow urban lakes are prone to hydrodynamic stagnation due to limited water exchange, rendering them highly susceptible to eutrophication and algal blooms However, for small urban lakes with weak hydrodynamics, key knowledge gaps remain: whether water diversion can disrupt stagnant zones, what critical flow rate is required to suppress algae, and which regions are most suitable for submerged macrophyte restoration. Focusing on Lianshi Lake-a small shallow urban lake in North China-this study developed a coupled hydrodynamic-water quality-ecology model based on the Environmental Fluid Dynamics Code (EFDC) to quantify how water diversion and submerged macrophyte restoration regulate circulation patterns and algal biomass under weak hydrodynamic conditions. Water diversion induced a pronounced nonlinear response in algae: a critical discharge of approximately 20 m3/s was identified, marking the transition of chlorophyll-a (Chl-a) concentrations from rapid decline to a stable low level. This critical discharge coincided with the minimum number of closed circulation zones, indicating that lake-wide algal suppression is closely linked to the mitigation of hydrodynamic stagnation. High Chl-a concentrations consistently occurred in low-velocity areas across different months, underscoring circulation structure as a key driver of spatial heterogeneity in algal distribution. In contrast, at the tested restoration scale, submerged macrophyte planting did not restructure the lake-wide flow field; its hydrodynamic impacts were confined to local regions. The Chl-a response to macrophyte restoration exhibited spatial heterogeneity, the most significant Chl-a reduction was observed in the western nearshore zone-characterized by weak water exchange and high background Chl-a concentrations-while improvements were limited in areas with favorable exchange conditions. Overall, the findings support the integration of water diversion strategies (to enhance hydrodynamic exchange) and targeted macrophyte restoration to mitigate algal bloom hotspots in eutrophic small shallow urban lakes. However, further validation in larger lake systems is warranted to generalize these conclusions.