Alternative dynamic regimes of marine biogeochemical models 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 state-of-the-art biogeochemical model. 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 alternative regimes exist and that the biogeochemical cycles commonly respond reversibly to the perturbation of the environment, i.e. when the perturbation is removed the original regime is recovered. A single forcing, depletion of nutrients, induced hysteresis in the dynamic regimes associated with changes in the planktonic trophic web, which sustains the biogeochemical cycles. The large number of numerical simulations, with a 1D water-column physical-biogeochemical model, under a vast range of environments and methodologies (sequential simulations, initial condition perturbation and 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 permitted us to identify possible dangerous path of the ocean state under the future climate, such as the hysteretical response under the changes of nutrient influx.