Alternative dynamic regimes in marine biogeochemical cycles under extreme environments

Guido Occhipinti1,2*, Davide Valenti3 and Paolo Lazzari1,2

1 National Institute of Oceanography and Applied Geophysics - OGS, via Beirut 2, Trieste, I-34014, Italy

2 NBFC, National Biodiversity Future Center, Palermo, I-90133, Italy

3 Dipartimento di Fisica e Chimica ""Emilio Segrè"" - Università degli Studi di Palermo, Viale delle Scienze, Ed. 18, Palermo, I-90128, Italy

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 in the Mediterranean Sea. 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. These results were supported by a large number of numerical simulations and a sensitivity analysis of the model parameters, which confirmed 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.