How Intensified Oceanic Transports Shift Organic Carbon Pathways in the Benguela Upwelling System

The Benguela Upwelling System (BUS) is one of the most productive marine ecosystems globally, providing vital ecological services and economic values. Under global warming conditions, upwelling intensification is expected to enhance the vertical ascent of nutrient-rich deep waters, fueling primary production. However, this intensification may also increase offshore lateral advection, potentially counteracting coastal ecosystem productivity. Enhanced offshore transport could reduce phytoplankton availability in coastal zones, affecting zooplankton and higher trophic levels, and weaken microbial activity by altering organic carbon sinking to deeper layers. This study quantifies the extent of Total Organic Carbon (TOC) lateral transport under varying upwelling conditions and examines associated changes in organic carbon pathways within the marine food web.

We employ a coupled physical-biogeochemical modeling system based on the Nucleus for European Modeling of the Ocean (NEMO v4.2.2) and the Biogeochemical Flux Model (BFM v5.3). A regional model configuration, encompassing the larger BUS domain at a 1/16^° horizontal resolution, was nested into a global ocean model at 1/4^° resolution using a two-way nesting approach. The BFM explicitly resolves pelagic-benthic coupling through sediment remineralization and adopts an intermediate-complexity structure to describe lower trophic-level ecosystem dynamics. The coupled model simulation spans 1980–2020, driven by ERA5 atmospheric forcing and GLOFASv2.1 runoff data.

Our results reveal that offshore lateral TOC transport in the upper 200 m of the Benguela region has steadily increased over the past four decades, with rates surpassing 2 mg C·m^⁻²·s^⁻¹ per decade. This trend reflects intensified coastal upwelling dynamics, which was more significant in the northern subregion during the austral spring season. Additionally, the displacement of organic matter export in the epipelagic zone towards the offshore open ocean highlights shifts in organic carbon pathways. These changes have also impacted the standing stocks of living and non-living groups within the lower trophic ecosystem, influencing carbon sinking dynamics across different pelagic zones.