A Large-scale mesocosm experiment was conducted in the temperate fjord ecosystem near Bergen, Norway during late spring 2022 to investigate the effects of OAE on biogeochemical processes and organisms under close-to-natural conditions. 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. Carbonate chemistry parameters were monitored for the total length of the 53-day experiment. The added TA remained relatively stable over the experimental period and the measured CO2 gas exchange rate was comparable to what would be expected for large oceanic regions. The study and data set offer insights into impacts of non-equilibrated alkalinity enhancement on the carbonate system, air-sea gas exchange and carbon fixation.This dataset is included in the OA-ICC data compilation maintained in the framework of the IAEA Ocean Acidification International Coordination Centre (see https://oa-icc.ipsl.fr). Original data were downloaded from the PANGAEA dataset (see Source). In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2024) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2026-05-26.