Long-Term Changes in Oxygen Dynamics in the Northern Benguela Upwelling System

The Benguela Upwelling System (BUS) is one of the world’s most productive marine ecosystems, driven by wind-induced upwelling that delivers nutrient-rich deep waters to the surface, fueling high primary productivity. Dissolved oxygen (DO) in the BUS exhibits a pronounced latitudinal gradient, with the northern BUS (NBUS, 15°S–23°S) persistently hypoxic (<60 mmol O₂/m³), in stark contrast to the well-oxygenated southern BUS. Climate warming intensifies these challenges, as rising ocean temperatures, stronger upwelling, and increased oxygen consumption drive reductions in DO levels, threatening fisheries, benthic biodiversity, and ecosystem services.

This study addresses key questions on the long-term changes in DO dynamics in the NBUS: (1) What is the long-term variability in the total DO inventory over the past four decades (1980–2020)? (2) What are the primary drivers of oxygenation and deoxygenation trends? (3) What role does SACW play in regulating oxygen levels? (4) How does the OMZ vary in spatial extent, volume, and boundary depth over time? (5) What are the relative contributions of physical and biogeochemical processes to DO variability?

To investigate these questions, we employ a coupled physical-biogeochemical modeling system based on the Nucleus for European Modeling of the Ocean (NEMO v4.2.2) coupled with the Biogeochemical Flux Model (BFM v5.3). The model, with a horizontal resolution of 1/16° (~7 km), simulates pelagic-benthic interactions, lower trophic-level dynamics, and sediment remineralization, as driven by ERA5 atmospheric reanalysis and GLOFASv2.1 river discharge data.

Our results reveal a vertical dipole in oxygen trends. Positive trends dominate the upper 200 meters, linked to a declining SACW fraction (0–400 meters), while negative trends at 400–950 meters are primarily driven by ocean warming. OMZs show contrasting patterns, with the threshold OMZ (<120 mmol O₂/m³) expanding and the core OMZ (<20 mmol O₂/m³) contracting. This study highlights the complex interplay between warming, upwelling intensification, and ocean circulation in shaping oxygen dynamics in this highly productive marine system.