Description:
<jats:title>Abstract</jats:title><jats:p>Eastern Boundary Upwelling Systems (EBUS) are highly productive ecosystems. However, being poorly sampled and represented in global models, their role as atmospheric CO<jats:sub>2</jats:sub> 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 CO<jats:sub>2</jats:sub> partial pressure (pCO<jats:sub>2</jats:sub>) and outgassing in the entire system, but is exceeded in the south through biologically-mediated CO<jats:sub>2</jats:sub> uptake through biologically unused, so-called preformed nutrients supplied from the Southern Ocean. Vice versa, inefficient nutrient utilization leads to preformed nutrient formation, increasing pCO<jats:sub>2</jats:sub> and counteracting human-induced CO<jats:sub>2</jats:sub> invasion in the Southern Ocean. However, preformed nutrient utilization in the BUS compensates with ~22–75 Tg C year<jats:sup>−1</jats:sup> for 20–68% of estimated natural CO<jats:sub>2</jats:sub> outgassing in the Southern Ocean’s Atlantic sector (~ 110 Tg C year<jats:sup>−1</jats:sup>), implying the need to better resolve global change impacts on the BUS to understand the ocean’s role as future sink for anthropogenic CO<jats:sub>2</jats:sub>.</jats:p>