KOSMOS 2021 Gran Canaria mesocosm study on ocean alkalinity enhancement: phytoplankton metabolic rates

Ocean Alkalinity Enhancement (OAE) could augment long-term carbon storage and mitigate ocean acidification by increasing the bicarbonate ion concentration in ocean water. However, the side effects and/or potential co-benefits of OAE on natural planktonic communities remain poorly understood. To address this knowledge gap, 9 mesocosms were deployed in the oligotrophic waters of Gran Canaria, from September 14th to October 16th, 2021. A CO2-equilibrated Total Alkalinity (TA) gradient was employed in increments of 300 µmol·L-1, ranging from ~2400 to ~4800 µmol·L-1. The carbonate chemistry conditions in terms of TA and Dissolved Inorganic Carbon (DIC), which were then used to calculate pCO2 and pH, and the nitrate+nitrite, phosphate and silicate concentrations were measured every two days over the course of the 33-day experiment alongside the following biotic parameters. Net Community Production (NCP), Gross Production (GP), Community Respiration (CR) rates, as well as the metabolic balance (GP:CR), were monitored every two days through oxygen production and consumption using the winkler method. Fractionated 14C uptake and chlorophyll a were also determined every four days although, initially, the total PO14C and DO14C production were also measured every 4 days, in between, up to day 13. Finally, flow cytometry was also carried out every two days and synecococcus, picoeukaryote and nanophytoplankton abundances were obtained. No damaging effect of CO2-equilibrated OAE in the range applied here, on phytoplankton primary production, community metabolism and composition could be inferred from our results. 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.