Clearing floating submerged vegetation leaves: An effective management to stabilize the clear state in shallow lakes?

Abstract

Achieving sustainable clear states in eutrophic shallow lakes is challenging due to the lag between nutrient load reductions and ecosystem response, often resulting in regime shifts. Submerged vegetation tends to fall off and float to the surface and block light due to the instability of freshly restored lakes, a key feature in influencing whether lakes deteriorate again. However, the mechanisms linking such transient shading to regime shifts remain unclear. This study conducted in situ experiments that quantified the shading effect of floating submerged vegetation leaves. We introduced the novel parameters, light interception coefficients and function that served as a crucial link between experimental findings and numerical models. Notably, we developed an innovative module specifically designed to assess the impacts of different clearing measures on aquatic ecosystems, which had been seamlessly integrated into the PCLake model. This practical model was applied to Xinglong Lake, recently ecologically restored, to simulate variations in key ecological indicators (total phosphorus (TP), total nitrogen (TN), chlorophyll-a (Chl-a), submerged vegetation biomass (DVeg)) and identify regime shift thresholds under different nutrient loads, initial time and time intervals of leaf clearing. The experimental results showed that light interception coefficients exhibited a subtle pattern, initially increasing slightly with water depth before declining, ranging from 0.573 m²/kg to 0.982 m²/kg for Vallisneria natans. The scenarios simulations demonstrated that prolonging clearing intervals from 0 to 120 days resulted in elevated TP, TN, and Chl-a concentrations, accompanied by a decline in DVeg, even causing the lake to a turbid state. Resuming daily clearing after a period of cessation proved ineffective in restoring the lake ecosystem if a regime shift had occurred. As nutrient loads and interception coefficients increased, the time intervals for triggering regime shifts shortened. We conservatively recommended that leaf clearing intervals should not exceed 10 days and ideally begin by March to ensure sufficient light for submerged vegetation. The study provides valuable insights into the impact of transient shading from floating leaves on regime shifts and offers scientific guidance for maintaining shallow lakes sustainably clear.