Sustained plankton blooms under open chaotic flows

Abstract

We consider a predator–prey model of planktonic population dynamics, of excitable character, living in an open and chaotic fluid flow, i.e., a state of fluid motion in a region from which fluid trajectories are continuously escaping, but such that it contains a chaotic subset. During the time trajectories spend in this chaotic area they are characterized by a positive maximum Lyapunov exponent, so that close fluid particles separate exponentially. Despite that excitability in the predator–prey dynamics is a transient phenomenon and that fluid trajectories are continuously leaving the system, there is a regime of parameters where the excitation remains permanently in it, given rise to a persistent plankton bloom. This regime is reached when the time scales associated to fluid stirring become slower than the ones associated to biological growth.