Noise-triggered regime shifts in a simple aquatic model

We examined the effects of random fluctuations and diffusion on regime shifts in bistable aquatic ecosystems, using a simple two-component reaction-diffusion model with nutrients and phytoplankton. The corresponding mean-field model generates two stable attractors: one is a nutrient-dominated (ND) state or a clear-water state, and the other is a phytoplankton-dominated (PD) state or a turbid-water state. According to our simulations of the reaction-diffusion model without fluctuations, either of two stable states can be dominant corresponding to the given parameters and the configuration of two states, meaning that the long-term coexistence of these two states is difficult. The dominant state expands gradually eliminating the other non-dominant one, which results in the homogeneous distribution of the dominant state in the whole lake. Further simulations including the effects of temporally changing fluctuations show that the regime shifts driven by external noise proceed even within a bistable region, which complicates the detection of hysteresis in real aquatic ecosystems.