Responses of microbial metabolic rates to non-equilibrated silicate- versus calcium-based ocean alkalinity enhancement

Abstract. This study contributes to the inaugural exploration of non-equilibrated ocean alkalinity enhancement (OAE). Total alkalinity (TA) was manipulated, with silicate- and calcium-based ΔTA gradients ranging from 0 to 600 µmol L−1, without prior CO2 sequestration, under natural conditions and at a mesocosm scale (∼ 60 m3). This manipulation led to a sustained increase in pH and a decrease in pCO2 throughout the experiment, as full natural equilibration through sea–air gas exchange did not occur. Implemented in a neritic system under post-bloom conditions, a midway mixing event was simulated. After the inorganic nutrient addition, mild delays in bloom formation were observed. These delays were related to, though not directly proportional to, the ΔTA gradient, as indicated by the gross production (GP), net community production (NCP), and chlorophyll a (Chl a) concentrations. Notably, the delay was more pronounced for the calcium treatment set compared to the silicate one, with the low-TA treatments exhibiting earlier responses than the high-TA ones. This delay is likely due to the previously documented species-specific negative relationships between high pH/low pCO2 conditions and phytoplankton growth rates. This study underscores the need for further investigation into the implications of these response patterns in terms of trophic transfer and seasonal suitability. Moreover, it is anticipated that a greater delay in bloom formation would be evident with a larger non-equilibrated TA gradient, highlighting the importance of exploring variations in TA thresholds for a comprehensive understanding of OAE's impacts.