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How would ocean fertilization increase carbon dioxide uptake by phytoplankton?

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Illustration of various geoengineering techniques to address climate change, including carbon capture and storage methods like artificial trees, industrial scrubbers, and deep ocean CO2 storage, as well as solar reflectance strategies such as sulfur aerosols, reflective crops, cloud whitening, and orbital mirrors. The scene shows these interventions applied to land, sea, and atmosphere, with labeled features explaining each process. — Geoengineering Various geoengineering proposals designed to increase solar reflectance or capture and store carbon.

Ocean fertilization is a proposed geoengineering technique that aims to enhance the natural ability of phytoplankton to absorb carbon dioxide from the atmosphere.21

The process works by introducing nutrients—specifically iron or nitrates—into surface waters where primary productivity is currently low. By stimulating these blooms, the phytoplankton photosynthesize atmospheric carbon dioxide into new organic tissues.21

The intended mechanism for long-term carbon sequestration is that, upon death, these phytoplankton sink to the ocean floor, effectively transporting the carbon they absorbed out of the surface cycle. Ideally, a portion of this organic material would eventually be stored as sedimentary rock. However, the effectiveness of this method remains a subject of scientific debate, as some studies suggest that much of the carbon may be consumed by zooplankton near the surface or returned to the atmosphere through upwelling rather than reaching the seafloor.21