Eastern Boundary Upwelling Systems (EBUS) are highly productive ecosystems. However, being poorly sampled and represented in global models, their role as atmospheric CO2 sources and sinks remains elusive. In this work, we present a compilation of shipboard measurements over the past two decades from the Benguela Upwelling System (BUS) in the southeast Atlantic Ocean. Here, the warming effect of upwelled waters increases CO2 partial pressure (pCO2) and outgassing in the entire system, but is exceeded in the south through biologically-mediated CO2 uptake through biologically unused, so-called preformed nutrients supplied from the Southern Ocean. Vice versa, inefficient nutrient utilization leads to preformed nutrient formation, increasing pCO2 and counteracting human-induced CO2 invasion in the Southern Ocean. However, preformed nutrient utilization in the BUS compensates with ~22–75 Tg C year−1 for 20–68% of estimated natural CO2 outgassing in the Southern Ocean’s Atlantic sector (~ 110 Tg C year−1), implying the need to better resolve global change impacts on the BUS to understand the ocean’s role as future sink for anthropogenic CO2.

The Benguela Upwelling System in the southeast Atlantic Ocean is of crucial socio-economic importance due to its high productivity. However, predicting its response to global change and understanding past changes are still great challenges. Here, we compile data obtained from a research cruise and an oceanographic mooring to demonstrate that a topographically steered nutrient trapping zone develops in a narrow belt along the coast during the main upwelling season in austral spring and summer in the southern Benguela Upwelling System. High nutrient concentrations within this zone increase the impact of upwelling on the productivity of the southern Benguela Upwelling System, but the efficient nutrient trapping operates at the expense of decreasing oxygen concentrations. This enhances the probability of anoxic events emerging toward the end of the upwelling season. However, at the end of the upwelling season, the front that separates the coastally trapped waters from open shelf waters weakens or even collapses due to upwelling cessation and the reversing current regime. This, in addition to a stronger vertical mixing caused by winter cooling, fosters the ventilation of the nutrient trapping zone, which reestablishes during the following upwelling season. The postulated intensification of upwelling and changes in the ecosystem structure in response to global warming seem to reduce the nutrient trapping efficiency by increasing offshore advection of surface waters and plankton blooms. The intensified upwelling and resulting lower biological oxygen consumption appears to mask the expected impacts of global warming on the oxygen minimum zone (OMZ) in the southern Benguela Upwelling System. In contrast to other OMZs, including those in northern Benguela Upwelling Systems, the OMZ in the southern Benguela Upwelling System reveals so far no detectable long-term decrease in oxygen. Thus, the nutrient trapping efficiency seems to be a critical feature mitigating global change impacts on the southern Benguela Upwelling System. Since it is topographically steered, regional impacts on the nutrient trapping efficiency appear also to explain varying responses of upwelling systems to global change as the comparison between southern and northern Benguela Upwelling System shows. This emphasizes the need for further and more comparable studies in order to better understand the response of Eastern Boundary Upwelling Systems and their ecosystem services to global change.

Eastern Boundary Upwelling Systems (EBUS) are well-known for their high productivity and fishery yields. However, being scarcely sampled and poorly represented in global models, their role as CO2 sources and sinks to the atmosphere remains elusive. Here, we present a compilation of shipboard measurements over the past two decades, showing how the Benguela Upwelling System (BUS) in the southeast Atlantic Ocean acts as a CO2 source in the north and CO2 sink in the south. Surface warming of upwelled waters increases the partial pressure of CO2 (pCO2) and outgassing in both regions, but in the south, the biologically-mediated drawdown of CO2 exceeds this warming effect. Here, the biological carbon pump owes its stronger impact on pCO2 to higher shares of upwelling source waters carrying preformed nutrients supplied from the Southern Ocean. Their formation increases pCO2 in surface waters and counteracts human-induced invasion of CO2 in the Southern Ocean. However, their utilization in the BUS compensates for over 20% of the CO2 loss occurring in the Atlantic sector of the Southern Ocean. This emphasizes the role of the BUS as key to improve our understanding of the ocean’s response to climate change and the future evolution of CO2 in the atmosphere.

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