Effect of water quality and quantity regulation on regime shift between phytoplankton and macrophyte in shallow lakes

There are two stable states in shallow lake ecosystem: clear water state dominated by macrophyte, and turbid water state dominated by phytoplankton. Maintaining steady clear water state dominated by macrophyte is a key treatment for lake eutrophication. In Baiyangdian Lake, coverage of submerged plant and concentration of total phosphorus are evaluation index of steady-state type. Driving mechanism model was established to explore steady-state change. Influent phosphorus concentration and water residence time are important parameters affecting conversion between phytoplankton and macrophyte. Influent phosphorus concentration and water residence time could effectively change conversion and determine steady-state results. When influent phosphorus concentration is kept above 0.19 g·m⁻³ for long, lake will eventually evolve into a steady-state type of phytoplankton. Reducing water residence time in lake was conducive to evolution to macrophyte-type. Maximum water residence times corresponding to different influent phosphorus concentrations were calculated. With proper control of influent phosphorus concentration, adjustment of water residence time according to phosphorus concentration, control of average outflow, lake could retain macrophyte-type clear water state for a long time. This will provide some decision-making basis for ecological operation at Baiyangdian Lake water conservancy project.