Accounting for Microbial–DOC Feedbacks Reveal Ecological Constraints on Artificial Upwelling for Atmospheric Carbon Dioxide Removal

Abstract. Limiting global warming to 1.5–2 °C has intensified interest in marine carbon dioxide removal (mCDR) to offset residual CO2 emissions. Artificial upwelling (AU), which pumps nutrient-rich deep waters to the surface to stimulate biological production and enhance carbon export, has primarily been evaluated with respect to particulate organic carbon export. However, the response of dissolved organic carbon (DOC) and its microbial remineralisation remains poorly constrained, although AU has the potential to alter both DOC production and consumption due to nutrient input from subsurface waters. Here, we use an Earth system model with explicit representation of microbial DOC remineralisation to quantify the response of global DOC inventories and air–sea CO2 fluxes to AU. We perform global simulations in which artificial upwelling (AU) differently affects nutrient limitation for phytoplankton and heterotrophic bacteria. Since the role of iron limitation in bacterial DOC uptake is poorly constrained, we explore a full factorial set of ecological scenarios in which AU confers a competitive advantage to phytoplankton, bacteria, both, or neither. AU alters surface DOC inventories by shifting the balance between phytoplankton DOC production and bacterial-DOC degradation. When phytoplankton compete more strongly for nutrients than heterotrophs, DOC inventories increase; conversely, stronger bacterial competition reduces DOC stocks. These differences are quantitatively significant for regulating air–sea CO2 uptake under AU. Our results highlight microbial nutrient competition as a key ecological control on DOC-mediated carbon storage, underscoring the need to better constrain phytoplankton–bacteria interactions to assess the efficacy of AU as a carbon dioxide removal strategy.