Air-sea gas exchange measurements helped derive in-situ organic and inorganic carbon fixation in response to Ocean Alkalinity Enhancement in a temperate plankton community

Abstract. Ocean Alkalinity Enhancement (OAE) is a carbon dioxide removal strategy that aims to chemically sequester atmospheric CO₂ 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 reduced CO₂ partial pressures (pCO₂) and high pH. To investigate the effects of OAE on biogeochemical processes and organisms under close-to-natural conditions, a large-scale mesocosm experiment was conducted in the temperate fjord ecosystem near Bergen, Norway during late spring. A non-CO₂-equilibrated approach was chosen, simulating OAE with calcium- and silicon-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-day experimental period and the measured CO₂ gas exchange rate was comparable to what would be expected for large oceanic regions. We estimated that full equilibration (95 %) for a ∆TA of 600 µmol kg^-1 would take ~1050 days. Furthermore, there were a number of mineral-type and pCO₂/pH effects, with cumulative coccolithophore calcification showing an optimum curve response to decreasing pCO₂, consistent with findings from single-species laboratory cultures, while no mineral-type effect was observed. In contrast, in-situ net community production was higher in the silicate treatments but there was no pCO₂ effect. Zooplankton respiration, estimated from in-situ net community production and in-vitro net community production incubations, was lower for the silicate treatments and negatively correlated with pCO₂. 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.