Impacts of stochastic fluctuations on marine biogeochemical processes

Marine ecosystems, particularly plankton populations, play a fundamental role in biogeochemical cycles by regulating primary production, nutrient cycling, and carbon sequestration. These systems are subject to both deterministic forcings, such as seasonal temperature changes, and stochastic environmental variability, which can lead to significant fluctuations in plankton biomass. This thesis investigates how stochastic fluctuations influence marine biogeochemical processes, focusing on plankton dynamics within the framework of the Biogeochemical Flux Model (BFM). The research aims to disentangle the relative contributions of endogenous dynamics, arising from internal food web interactions like predation and competition, and exogenous stochastic forces, such as random environmental noise, to plankton variability. Many studies have been carried in the past within the framework of theoretical ecology considering simplified models such as the Generalized Lotka-Volterra models. The novelty of the present work is that the examined biogeochemical model has an higher degree of realism and complexity and it is used to provide operational forecasts and in the context of climate studies. Further, complex mathematical concepts are used to change the model formulation and analyze its outputs. The thesis is divided into three main studies, progressively increasing in complexity. First, the investigation into endogenous plankton dynamics without external forcing revealed that the system remains relatively stable, with non-stationary dynamics occurring only in low-complexity food webs or under extreme model parameters. This suggests that intrinsic processes alone cannot explain the full extent of variability observed in natural ecosystems. The predator-prey interactions commonly obtained with predator-prey models, such as Lotka-Volterra, indicate that oscillations are originated from endogenous dynamics, on the contrary, here we show that for a complex model the "most probable" dynamics leads to the steady state. Second, the introduction of environmental stochasticity into the model showed that even low levels of noise can enhance species coexistence and trigger stochastic resonance. However, stronger noise destabilizes the system, pushing it towards new equilibria or even species extinction. This emphasizes the importance of considering stochasticity when modeling marine ecosystems. Finally, the combined effects of deterministic seasonal forcing and stochastic forcing were examined. The interaction between these two forces produced complex dynamics, including chaos, which may explain the year-to-year variability in plankton populations commonly observed in nature. Determining the origin of complexity in the temporal variability of ecosystems is one of the open problems in ecology, and there is still no consensus on whether it can be characterized by chaos or noise. Our results suggest that the observed variability can be chaotic or noisy, depending on the temporal scale at which it is studied. The findings of this thesis highlight the critical role of both stochastic and deterministic processes in shaping marine ecosystems. They suggest that a nuanced approach, which integrates both types of influences, is essential for accurately predicting ecosystem behavior, particularly in the context of climate change. These results offer new insights into the stability and variability of marine biogeochemical cycles, with implications for the management and conservation of marine biodiversity.