pH levels and ocean acidification in the central-southern Black Sea

Ocean acidification in the Black Sea has been reported to progress at a faster rate than the Global Ocean. Thanks to the high level of alkalinity, low temperature, and high productivity, the Black Sea has a great potential to absorb and store atmospheric CO2 in the long term. However, the past studies carried out in the area are not homogeneous, and very little is known about the central-southern basin, which highly differs in terms of circulation and productivity from the northern part. In this study, we aimed to characterize the pH levels in the central and southern Black Sea, assess the spatial variability between the coast and offshore waters, and estimate the ocean acidification rates of the last decades. Samples were collected during a winter trans-Black Sea cruise in December 2022 (from the Bosphorus to Georgia) both at the surface, with a continuous underway system (SeaFET V2 Ocean pH sensor) and along the water columns, with discrete samples, later measured with the spectrophotometric method. Surface levels of the pH (8.40-8.50) presented a limited variability between the coastal and offshore waters when compared with other adjacent seas (Marmara and Mediterranean). On the other hand, the highest variability was observed along the water column where the pH varied between 7.78 and 8.50 due to the anoxic layer located below the euphotic layer. A comprehensive analysis of the historical pH data compared with measured pH levels seems to confirm the increasing trend of ocean acidification also in the central southern part of the Black Sea.