Alternative dynamic regimes of plankton communities in perturbed environments

Abstract. The existence of alternative dynamic regimes or equilibria has been widely observed in the biosphere and the climate system. In order to assess the potential impacts of climate change and develop effective mitigation and adaptation strategies, a comprehensive knowledge of these alternative regimes is crucial. We studied marine biogeochemical cycles, which are fundamental for sustaining ocean life and for climate regulation, with a biogeochemical model used for operational purposes. We investigated whether the perturbation of the environment (e.g. air temperature, wind velocity, nutrient input) to extreme values can push biogeochemical cycles into a different regime. We have established that this phenomenon exists and that the system commonly responds reversibly to the perturbation of the environment, i.e. when the perturbation is removed the original system regime is recovered. Depletion of nutrients and increase in wind velocity can induce hysteresis in the dynamic regimes associated with changes in the planktonic trophic web, which sustains the biogeochemical cycles. The large number of numerical simulations under a vast range of environments and methodology, comprising demographic stochasticity, underpins the generality of the results and the sensitivity analysis of the model parameters confirms the accuracy of the model even under extreme environments. The occurrence of alternative dynamic regimes in a modern marine biogeochemical model, supported by field observations of regime shifts in plankton, suggests its use in predicting the state of the ocean under climate change.