Response of the Carbonate System to Ocean Alkalinity Enhancement: A Mesocosm-Scale Experiment

We present the results of a mesocosm-scale experiment conducted in 2023 in the eastern Mediterranean Sea (Crete, Greece) to evaluate the effects of ocean liming via the addition of calcium hydroxide. We assessed two dosage regimes through repeated additionslow (20 μmol L^-1 alkalinity increase per addition, reaching a total theoretical alkalinity increment of 120 μmol L^-1) and high (180 μmol L^-1 per addition, reaching a total theoretical increment of 1080 μmol L^-1)comparing against control mesocosms over 14 days, monitoring the pH, total alkalinity, dissolved inorganic carbon, and solid precipitates. The results for the treated mesocosms show that the low dosage effectively increased the total alkalinity and maintained it over the duration of the experiment, unlike the high dosage mesocosms, where the alkalinity dropped within a few days due to the secondary precipitation of carbonates and degassing. In particular, the final net changes of alkalinity were +110 μmol L^-1 for low dosages (close to the theoretical limit) but amounted to about -330 μmol L^-1 for the high ones. The concurrent changes in pH were equal to +0.1 and +0.4, while those in dissolved inorganic carbon were equal to -60 and -620 μmol L^-1, respectively. The fact that, in the low dosage mesocosms, there was a slight reduction in the dissolved inorganic carbon (comparable to that occurring in control mesocosms) despite the total alkalinity increase suggests limited atmospheric CO₂ uptake. We attribute this to restricted air-sea gas exchange caused by the mesocosm lids, which are commonly adopted in these facilities to prevent the entrance of external contaminants. Our study highlights future directions for testing and enhancing the efficiency of CO₂ uptake by seawater, improving ocean liming as a viable climate mitigation strategy.