Marine productivity in Eastern boundary upwelling systems under climate change

The impact of climate change on the highly sensitive Eastern Boundary Upwelling Systems (EBUS) is currently a significant concern. These regions-California, Canary, Humboldt, and Benguela-contribute disproportionately to global marine productivity, supporting essential ecosystem services. Consequently, it is crucial to reduce the substantial uncertainties that still persist regarding the impact of climate change on these regions and on the ecosystems they support. This study utilizes scenario simulations (SSP5-8.5, SSP2-4.5, and SSP5-3.4-OS) from several Earth System Models of the CMIP6, in particular the latest model by IPSL (IPSL-CM6A-LR), to evaluate early theories and state-of-the-art hypotheses explaining the mechanisms driving changes in Net Primary Productivity (NPP). The findings reveal that the current understanding of these processes is partial, primarily accounting for large-scale atmospheric changes associated with Hadley cell expansion, which causes the poleward shift of upwelling regions and of NPP hotspots. While upwelling-favorable winds changes are recognized as significant drivers of NPP patterns, they alone cannot fully explain the projected changes in NPP across all regions. Through a correlative approach and a quantitative analysis of growth rates, this thesis emphasizes the central role in shaping productivity of temperature limitation and of changes in the concentrations of subsurface nutrients. The analysis of the overshoot scenario further highlights the importance of deep nutrient reservoirs in sustaining NPP within the euphotic layer. The study demonstrates the complexity of the subject, due to the intricate interplay of changes in upwelling intensity, source-water nutrient content, and other dynamical and biogeochemical processes, and highlights the potential ecological and economical impacts of changes in net primary productivity on marine ecosystems.