A conceptual model for predicting and managing vegetative types in shallow lakes

Abstract

The model presented is composed of two curved surfaces in a three-dimensional space. The first dimension is the external nutrient loading of the lake (ENL). Water nutrient level (WNL) is defined as the second dimension. The third represents relative dominance of macrophytes and phytoplankton (RD). The upper surface represents macrophyte-dominated clear-water phase, and the lower surface represents phytoplankton-dominated turbid-water phase. The two phases are separated into six states (M1, M2, M3, P1, P2, P3) by the critical values of a and b on the WNL dimension. When WNL< a, there is only one stable state, macrophyte-dominated clear-water state M1. Some biotic or environmental factors may keep the lake at a turbid state, a sub-stable state P1. If these factors are removed, the system will automatically skip to the M1 state. When a< WNL< b, two stable states, M2 and P2, may exist. The M2 state may be maintained by macrophytes due to their positive effects on the lake water and their inhibition effects on phytoplankton. If macrophytes are destroyed for any reason, the system may irreversibly skip to the P2 state, phytoplankton-dominated turbid-water. To restore the system to the M2 state, much more help is needed for reducing phytoplankton and increasing water transparency. When WNL> b, there is only one stable state, the phytoplankton-dominated turbid-water state P3. An unstable state M3 may exist if b< WNL< c, but macrophyte dominance will be automatically replaced by phytoplankton dominance in a short time, and WNL will increase to c at the same time. When WNL> b, any attempt to restore the system by biomanipulation will be fruitless.