Non-equilibrated ocean alkalinity enhancement influences nitrogen retention and export

Ocean alkalinity enhancement (OAE) has been proposed as a carbon dioxide removal technology (CDR) allowing for long-term storage of atmospheric carbon dioxide (CO₂) in the ocean. By changing the carbonate speciation in seawater, OAE may alter plankton communities and the particle export they drive. Using mesocosms in the mesotrophic Raunefjord, Bergen (Norway), we employed five different alkalinity levels for each, a lime- and olivine-based OAE scenario. Total alkalinity (TA) was raised in increments of 150 µmol kg^-1­ (ΔTA_max = 600 µmol kg^-1) using NaOH solutions. Seawater pCO₂ was left to equilibrate with the atmosphere, leading to strong (pH_max = 8.80) but transient changes in carbonate chemistry. In concert with TA, CaCl₂ (lime-based) or MgCl₂ (olivine-based) was added to simulate the respective cation increase during mineral application. Additionally, equal amounts of Na₂SiO₃ (75 µmol L^-1)was added to all olivine-based mesocosms to simulate the release of silicate whilst separating it from TA effects. Here, we provide insights of the two different OAE approaches on the flux and attenuation of sinking particles. After 49 days of non-equilibrated OAE, the community-mediated cumulative export flux of major elements (POC, PON, POP, BSi) was higher in the olivine- compared to the lime-based application. Preferential remineralization of nitrogen over carbon within the export flux decreased with TA, suggesting a potential nitrogen loss to the surface ocean, potentially shortening productive bloom periods and thus decreasing export production of carbon. This potential negative feedback on atmospheric CO₂ levels under OAE warrants further investigation, specifically with respect to its dependence on plankton community composition, heterotrophic nitrogen remineralization, and the chosen alkalinity enhancement approach.