Assessing the impact of CO₂ equilibrated ocean alkalinity enhancement on microbial metabolic rates in an oligotrophic system

Abstract. Ocean Alkalinity Enhancement (OAE) is a Negative Emissions Technology (NET) that shows significant potential for climate change mitigation. By increasing the bicarbonate ion concentration in ocean water, OAE could enhance long-term carbon storage and mitigate ocean acidification. However, the side effects and/or potential co-benefits of OAE on natural planktonic communities remain poorly understood. To address this knowledge gap, a mesocosm experiment was conducted in the oligotrophic waters of Gran Canaria. A CO₂-equilibrated Total Alkalinity (TA) gradient was employed in increments of 300 µmol·L^-1, ranging from ~2400 to ~4800 µmol·L^-1. This study represents the first attempt to evaluate the potential impacts of OAE on planktonic communities under natural conditions. The results show that Net Community Production (NCP), Gross Production (GP), Community Respiration (CR) rates, as well as the metabolic balance (GP:CR), did not exhibit a linear response to the whole alkalinity gradient. Instead, significant polynomial and linear regression models were observed for all rates up to ∆TA1800 µmol·L^-1, in relation to the Dissolved Inorganic Carbon (DIC) concentrations. Notably, the ∆TA1500 and 1800 µmol·L^-1 treatments showed peaks in NCP shifting from a heterotrophic to an autotrophic state, with NCP values of 4 and 8 µmol O₂ kg^-1 d^-1, respectively. These peaks and the optimum curve were also reflected in the nanophytoplankton abundance, size-fractionated chlorophyll a and ¹⁴C uptake data. Furthermore, abiotic precipitation occurred in the highest treatment after day 21 but no impact on the measured parameters was detected. Overall, a damaging effect of CO₂-equilibrated OAE in the range applied here, on phytoplankton primary production, community metabolism and composition could not be inferred. In fact, a potential co-benefit to OAE was observed in the form of the positive curvilinear response to the DIC gradient up to the ∆TA1800 treatment. Further experimental research at this scale is key to gain a better understanding of the short and long-term effects of OAE on planktonic communities.