Role of Time-Varying Physical Forcing in Plankton Ecosystems Dynamics

In the past decades, the coupling of ocean physics and of ecosystem dynamics has been investigated at length, in particular in terms of dynamical systems, but the impact of physics on the evolution of the ecosystem has been less often studied mathematically. The present contribution addresses the influence of a time-varying physical forcing of the nutrients, on the evolution of a simple ecosystem with three trophic levels. The intrinsic dynamics of the ecosystem is computed analytically first and the influence of physics is then studied numerically. A harmonic forcing introduced in the presence of a limit cycle of the ecosystem leads to a resonance in its evolution. A sub-harmonic forcing introduces a resonance only for a strong perturbation. The phase shift of the forcing to the oscillation has an impact on the amplitude of the oscillations. Thus, close to marginality, the system is sensitive to the physics driving the input of nutrients.