Seasonal and regional pH variation determined from continuous spectrophotometric measurements on a ship of opportunity in a coastal region

Coastal regions are under the threat of surface water acidification, which its ecological and socioeconomic impacts need to be better constrained. However, pH measurements in coastal waters are challenging and are still primarily measured discretely, compromising spatial or temporal scales. Therefore, directly measuring an acidification parameter, such as pH, with high spatial and temporal coverage could improve our understanding of the changes in the water acid-base balance and, thus, potential changes in the biogeochemical processes in these highly dynamic regions. Contributing to coastal management, we analysed continuous surface spectrophotometric pH measured on board the Ship of Opportunity "Finnmaid" along the Baltic Sea over the year 2020. We observed a pronounced seasonality of isothermal (25 ^oC) pH (total scale, pHT), with higher pH values in warm seasons (8.206 ± 0.148) and lower in colder seasons (7.959 ± 0.065), with maximum (8.792) and minimum (6.971) pH observed in the Gulf of Finnland during the summer. Consistently, surface pCO₂ mirrored pH, with general averages of 299.6 ± 103.5 µatm (spring) and 279.4 ± 108.0 µatm (summer). In addition, the high-frequency measurements enable us to investigate biogeochemical processes at the submesoscale and, thus, better resolve sub-basin and sporadic coastal processes, such as river discharge and upwelling events. For this purpose, the Baltic Sea was sub-dived into three basins along the ship track: the German coast, the Gotland Sea, and the Gulf of Finland. Data in the Gulf of Finland indicated higher biological productivity during the warm season (spring-summer), depicted by a more significant surface pCO₂ drawdown (minimum of 181.2 µatm) compared to the Gotland Sea (237.3 µatm) and German coast (200.8 µatm) minima. Consequently, pH values followed this pattern, reaching maxima of 8.792, 8.433, and 8.562 in summer, respectively. The results indicate that seasonal pH variations are controlled mainly by biological processes, which, in turn, vary regionally due to differences in the external conditions (e.g., light availability), the hydrographical setting (e.g., temperature and water column structure) and nutrient availability. Furthermore, the high spatiotemporal resolution of pH measurements achieved here allows for tracking the minimum and maximum pH values encountered in the surface water over the entire year, which can support current efforts towards the development of an acidification indicator within HELCOM.