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

• Main Authors: Marín-Samper, Laura, Arístegui, Javier, Hernández-Hernández, Nauzet, Ortiz, Joaquín, Archer, Steve D., Ludwig, Andrea, Riebesell, Ulf
• Format: Text
• Language: English
• Published: 2024
• Subjects: Ocean acidification
• Online Access: https://doi.org/10.5194/egusphere-2023-2409; https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2409/

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 CO 2 -equilibrated total alkalinity (TA) gradient was employed in increments of 300 µ mol L −1 , ranging from ∼ 2400 to ∼ 4800 µ mol 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, and 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 Δ TA 1800 µ mol L −1 , in relation to the dissolved inorganic carbon (DIC) concentrations. Notably, the Δ TA 1500 and 1800 µ mol L −1 treatments showed peaks in NCP shifting from a heterotrophic to an autotrophic state, with NCP values of 4 and 8 µ mol O 2 kg −1 d −1 , respectively. These peaks and the optimum curve were also reflected in the nanoplankton abundance, size-fractionated chlorophyll a , and 14 C 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 CO 2 -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 Δ TA 1800 treatment. Further experimental research at this scale is key to ...