Carbon fixation of a temperate plankton community in response to calcium- and silicate-based Ocean Alkalinity Enhancement using air-sea gas exchange measurements

Ocean Alkalinity Enhancement (OAE) is a carbon dioxide removal strategy that aims to chemically sequester atmospheric CO 2 in the ocean while potentially alleviating localized effects of ocean acidification. Depending on the implementation approach, OAE can considerably alter seawater carbonate chemistry, resulting in temporarily reduced CO 2 partial pressure ( p CO 2 ) and elevated pH before re-equilibration with the atmosphere or mixing with unperturbed waters. To investigate the effects of OAE on biogeochemical processes and organisms under close-to-natural conditions, a large-scale mesocosm experiment was conducted in a temperate fjord ecosystem near Bergen, Norway, during late spring. A non-CO 2 -equilibrated OAE approach was chosen, simulating OAE with calcium- and silicate-based minerals. A gradient of five OAE levels was achieved by increasing total alkalinity (TA) by 0–600 µmol kg −1 . The added TA remained relatively stable over the 47 d experiment and measured CO 2 gas exchange rates reached up to −15 mmol C m −2 d −1 . We estimated that full equilibration (95 %) by air-sea gas exchange for a ΔTA of 600 µmol kg −1 would take ∼1050 d. Furthermore, various mineral-type and/or p CO 2 / pH effects were found. Coccolithophore calcification followed an optimum curve response along the p CO 2 gradient, consistent with findings from single-species laboratory cultures. In contrast, in-situ net community production (NCP) was higher in the silicate-based treatments, but was not modified by changes in p CO 2 . Zooplankton respiration, estimated from in-situ NCP and in-vitro NCP incubations, was lower for the silicate-based treatments and negatively correlated with p CO 2 . These complex findings suggest both direct and indirect effects of mineral type and OAE level and provide a valuable foundation for designing future OAE field trials. For a safe application of OAE, non-equilibrated alkalinity additions must balance efficiency and environmental impact.