Interactions between ocean alkalinity enhancement and phytoplankton in an Earth System Model

Abstract. Ocean alkalinity enhancement (OAE) as a CO₂ removal strategy is well investigated in model studies, but risks for the ecosystem are presently not considered in models. Our study examines OAE-phytoplankton feedbacks in an Earth System Model by adding carbonate system dependencies to the phytoplankton growth term. OAE is performed between 2040 and 2100 in the exclusive economic zones of Europe, the US, and China, with alkalinity additions reaching 103.2 Tmol year⁻¹ by the end of the century. Atmospheric pCO₂ is reduced by 3–8 µatm. The excess ocean CO₂ sink is mainly chemically driven, but can additionally be altered by biological feedbacks. Further, net primary production decreases by up to 15 % due to indirect effects of OAE. Our results do not confirm the direct positive effect of OAE on calcifying coccolithophores. Limiting alkalinity addition in locations with high aragonite saturation states is beneficial as it not only reduces the OAE impact on phytoplankton but also increases the reduction in atmospheric pCO₂. Our study highlights the need to take ecosystem responses into account when evaluating the effectiveness of OAE.